868 lines
17 KiB
Plaintext
868 lines
17 KiB
Plaintext
#LyX 2.1 created this file. For more info see http://www.lyx.org/
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\shortcut idx
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\end_header
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\begin_body
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\begin_layout Title
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PVAccess Gateway Design
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\end_layout
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\begin_layout Author
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Michael Davidsaver
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\end_layout
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\begin_layout Date
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\begin_inset ERT
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status collapsed
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\begin_layout Plain Layout
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\backslash
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today
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Section
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Scope
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\end_layout
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\begin_layout Standard
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A PVAccess gateway will act as a
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\begin_inset Quotes eld
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\end_inset
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switch
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\begin_inset Quotes erd
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\end_inset
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to route PVA traffic between IP subnets.
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The primary use cases for a gateway are: 1) de-duplicate data when many
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PVA clients make a similar request to limit the load on the PVA server
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handling this request.
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And 2) to provide the mechanisms to implement site specific access control
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policies specifying which requests are permitted.
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\end_layout
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\begin_layout Standard
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\begin_inset Float figure
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wide false
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sideways false
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status collapsed
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\begin_layout Plain Layout
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\align center
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\begin_inset Graphics
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filename parts.dia
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height 1in
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\end_inset
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\end_layout
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\begin_layout Plain Layout
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\begin_inset Caption Standard
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\begin_layout Plain Layout
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Gateway Components
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\end_layout
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\end_inset
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\begin_inset CommandInset label
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LatexCommand label
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name "fig:parts"
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\end_inset
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Section
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Terms
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\end_layout
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\begin_layout Description
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IP Internet Protocol (v4 and/or v6)
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\end_layout
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\begin_layout Description
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PV Process Variable.
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Addressable unit in PVA.
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A character string.
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\end_layout
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\begin_layout Description
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PVA PVAccess network protocol
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\end_layout
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\begin_layout Description
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GW Gateway
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\end_layout
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\begin_layout Description
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CLI An arbitrary PVAccess client (end user or another gateway)
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\end_layout
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\begin_layout Description
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GWS Gateway server side (CLI communicates with this)
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\end_layout
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\begin_layout Description
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GWC Gateway client side (SRV communicates with this)
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\end_layout
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\begin_layout Description
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SRV An arbitrary PVAccess server (may be another gateway)
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\end_layout
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\begin_layout Standard
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Figure
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "fig:parts"
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\end_inset
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shows the relationships between the CLI, GWS, GWC, and SRV actors.
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The
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\begin_inset Quotes eld
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\end_inset
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Gateway
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\begin_inset Quotes erd
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\end_inset
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consists of the GWS, GWC, and the logic which sits between them.
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\end_layout
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\begin_layout Section
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Goals/Features
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\end_layout
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\begin_layout Standard
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Identified features and design goals.
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\end_layout
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\begin_layout Paragraph
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De-duplication
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\end_layout
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\begin_layout Standard
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of sockets and data to reduce overall resource use.
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\end_layout
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\begin_layout Paragraph
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Policies
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\end_layout
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\begin_layout Standard
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to permit or deny individual requests.
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This include access control policy (ie.
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client
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\begin_inset Quotes eld
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\end_inset
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X
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\begin_inset Quotes erd
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\end_inset
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may not access resource
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\begin_inset Quotes eld
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\end_inset
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Y
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\begin_inset Quotes erd
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\end_inset
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) and administrative limits to bound per-client resource usage (ie.
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a client may have only 15 concurrent
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\begin_inset Quotes eld
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\end_inset
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gets
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\begin_inset Quotes erd
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\end_inset
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in progress).
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\end_layout
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\begin_layout Section
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Theory of Operation
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\end_layout
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\begin_layout Subsection
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Name resolution/socket setup
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\end_layout
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\begin_layout Standard
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Use of periodically re-sent UDP messages by PVA for name search permits
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an asynchronous mode of operation for name resolution and socket (circuit)
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setup.
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\end_layout
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\begin_layout Standard
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\begin_inset listings
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lstparams "language={C++}"
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inline false
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status open
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\begin_layout Plain Layout
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struct ChannelCacheEntry {
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\end_layout
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\begin_layout Plain Layout
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string name; // key
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\end_layout
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\begin_layout Plain Layout
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int priority; // key
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\end_layout
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\begin_layout Plain Layout
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unsigned refcount; // implicily via.
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shared_ptr
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\end_layout
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\begin_layout Plain Layout
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bool searched;
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\end_layout
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\begin_layout Plain Layout
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shared_ptr<Channel> chanGWC;
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\end_layout
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\begin_layout Plain Layout
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};
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Paragraph
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A cache of GWC Channels
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\end_layout
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\begin_layout Standard
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index by name and priority (the parameters of the createChannel message)
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is maintained.
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Entries in this ChannelCache have an associated GWC Channel, and a garbage
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cleanup flag.
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\end_layout
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\begin_layout Standard
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When GWS receives a new CLI search request message a lookup is made to the
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ChannelCache which has three possible outcomes (see figure
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "fig:name-search"
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\end_inset
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).
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\end_layout
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\begin_layout Standard
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\begin_inset Float figure
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wide false
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sideways false
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status collapsed
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\begin_layout Plain Layout
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\align center
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\begin_inset Graphics
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filename name-search.msc
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width 3in
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height 2in
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keepAspectRatio
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\end_inset
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\end_layout
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\begin_layout Plain Layout
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\begin_inset Caption Standard
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\begin_layout Plain Layout
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CLI name search outcomes
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\end_layout
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\end_inset
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\begin_inset CommandInset label
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LatexCommand label
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name "fig:name-search"
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\end_inset
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Description
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Miss There is no entry in the cache.
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A new entry is created.
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The new GWC Channel begins the search/connect process.
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No reply to CLI is made.
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\end_layout
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\begin_layout Description
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Not
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\begin_inset space ~
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\end_inset
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Conn An entry exists, but the associated GWC Channel is not yet connected.
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No further action is taken.
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No reply to CLI is made.
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\end_layout
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\begin_layout Description
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Hit An entry exists with a connected GWC Channel.
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A positive search reply is sent to CLI.
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\end_layout
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\begin_layout Standard
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Handling of TCP channelCreate messages by GWS is similar except that a negative
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reply is sent for Miss and Not Conn outcomes.
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A negative reply to a channelCreate message by CLI will cause it to restart
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the name search phase.
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\end_layout
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\begin_layout Paragraph
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A reference count
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\end_layout
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\begin_layout Standard
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is maintained by each ChannelCache entry for the GWS channels using the
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associated GWC channel.
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However, an entry should not be immediately removed when its ref.
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count drops to zero.
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Instead it should be kept for some time so that it may be found by future
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CLI search requests.
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\end_layout
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\begin_layout Standard
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Each ChannelCache entry will have a boolean flag which is set on creation,
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and re-set whenever it is found by a name lookup (Miss or Not Conn).
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A periodic cleanup task should run which removes all entries with this
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flag cleared and a zero ref.
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count.
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Each time it run, the cleanup task clears the flag of any entry not removed.
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\end_layout
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\begin_layout Standard
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This should ensure that unused entries are eventually removed when no client
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is searching for them.
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\end_layout
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\begin_layout Paragraph
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Ownership of an active GWC Channel
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\end_layout
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\begin_layout Standard
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is shared by several active GWS channels, and a ChannelCache entry as shown
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in figure
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "fig:clichanown"
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\end_inset
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.
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Red arrows represent strong ownership and blue weak ownership.
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\end_layout
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\begin_layout Standard
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\begin_inset Float figure
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wide false
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sideways false
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status collapsed
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\begin_layout Plain Layout
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\align center
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in general
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\begin_inset Graphics
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filename structs.dot
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width 4in
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height 3in
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keepAspectRatio
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\end_inset
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\end_layout
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\begin_layout Plain Layout
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\begin_inset Caption Standard
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\begin_layout Plain Layout
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GW Channel ownership
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\end_layout
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\end_inset
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\begin_inset CommandInset label
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LatexCommand label
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name "fig:clichanown"
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\end_inset
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Paragraph
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Notification of loss of a GWC Channel
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\end_layout
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\begin_layout Standard
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should result in the disconnection of any associated GWS Channels, and the
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immediate removal of the associated ChannelCache entry.
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This allows the ref.
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count for an entry to fall to zero once all GWS Channels close.
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\end_layout
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\begin_layout Standard
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All pending operations associated with this channel will also be canceled.
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\end_layout
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\begin_layout Subsection
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Get/Put/RPC/...
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operations
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\end_layout
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\begin_layout Standard
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For operations other than Monitor, the timing of the client request can
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effect the results.
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No caching or de-duplication can be done without special knowledge about
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the intended behavior of CLI and SRV.
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Therefore, by default these operations pass through the GW without de-duplicati
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on.
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\end_layout
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\begin_layout Subsection
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Monitor operations
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\end_layout
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\begin_layout Standard
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Each ChannelCache entry will also include a MonitorCache.
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This cache is indexed by the pvrequest given with the corresponding monitorCrea
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te operation.
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As a pvrequest may contain arbitrary data, two pvrequests may not be compared
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for anything other than exact equality without special knowledge.
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Therefore, by default MonitorCache hits are only generated when the CLI
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provides a pvrequest which exactly matches the MonitorCache entry.
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\end_layout
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\begin_layout Standard
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Each MonitorCache entry should also keep the most recent value received
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by the GWC so that this may be returned immediately for new GWS subscriptions.
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\end_layout
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\begin_layout Standard
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A list of GWS consumers (interested in event data) will also be maintained.
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Each new event data value is passed into the MonitorRequester of each consumer.
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\end_layout
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\begin_layout Standard
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\begin_inset listings
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lstparams "language={C++}"
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inline false
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status open
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\begin_layout Plain Layout
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struct MonitorCacheEntry {
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\end_layout
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\begin_layout Plain Layout
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PVField request; // key
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\end_layout
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\begin_layout Plain Layout
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weak_ptr<ChannelCacheEntry> chan;
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\end_layout
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\begin_layout Plain Layout
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unsigned refcount; // implicily via.
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shared_ptr
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\end_layout
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\begin_layout Plain Layout
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shared_ptr<Monitor> mon;
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\end_layout
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\begin_layout Plain Layout
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PVField lastval;
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\end_layout
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\begin_layout Plain Layout
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list<MonitorConsumer*> consumers;
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\end_layout
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\begin_layout Plain Layout
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};
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\end_layout
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\begin_layout Plain Layout
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struct MonitorConsumer {
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\end_layout
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\begin_layout Plain Layout
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shared_ptr<MonitorRequester> queueGWS;
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\end_layout
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\begin_layout Plain Layout
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shared_ptr<MonitorCacheEntry> entry;
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\end_layout
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\begin_layout Plain Layout
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bool GC;
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\end_layout
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\begin_layout Plain Layout
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};
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Paragraph
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A reference count
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\end_layout
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\begin_layout Standard
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is maintained for each MonitorCacheEntry, which will be closed as soon as
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its ref.
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count falls to zero.
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\end_layout
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\begin_layout Section
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Channel Transmit Queuing
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\end_layout
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\begin_layout Standard
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A major potential pitfall of connection/socket sharing for distinct sub-channels
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(PVA Channels) is the handling of prioritization (or lack thereof) of traffic
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for different sub-channels.
|
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For example, monitoring a single high data rate PV can cause other PVA
|
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Channels to experience higher latency.
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One way to mitigate this, in part, is to introduce some
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
fairness
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
to the circuit/Transport transmit message queue.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Instead of a simple FIFO fed by all Channels, each channel given a FIFO.
|
|
The task which dequeues would do so by taking from each FIFO in turn in
|
|
a round robin.
|
|
This should prevent the overall latency through the queue from being dominated
|
|
by one fast PV.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
However, this is only a partial solution as PVs with a large data size can
|
|
still introduce latency in proportion to this size.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
For example, data updates from three source (A, B, and C) are added in one
|
|
order (line
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "line:queue1"
|
|
|
|
\end_inset
|
|
|
|
).
|
|
The
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
fair
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
queuing algorithm internally maintains a list of lists (line
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "line:queue2"
|
|
|
|
\end_inset
|
|
|
|
).
|
|
Round robin dequeuing from this list of lists gives the a different ordering
|
|
as seen on line
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "line:queue3"
|
|
|
|
\end_inset
|
|
|
|
.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset listings
|
|
lstparams "language=Python,numbers=left,escapechar={|}"
|
|
inline false
|
|
status open
|
|
|
|
\begin_layout Plain Layout
|
|
|
|
queue = [A, A, B, A, B, C, C, A, A, B]|
|
|
\backslash
|
|
label{line:queue1}|
|
|
\end_layout
|
|
|
|
\begin_layout Plain Layout
|
|
|
|
sort_fair(queue) # [[A, A, A, A, A], [B, B, B], [C, C]]|
|
|
\backslash
|
|
label{line:queue2}|
|
|
\end_layout
|
|
|
|
\begin_layout Plain Layout
|
|
|
|
queue == [A, B, C, A, B, C, A, B, A, A]|
|
|
\backslash
|
|
label{line:queue3}|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
Loop avoidance
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Another potential pitfall inherent in using UDP broadcasts for name resolution
|
|
is the possibility of loops should GWS receive search requests from GWC.
|
|
This can be avoided provided that GWS is aware of the set of endpoints
|
|
that GWC uses to send requests, and ignores an requests from them.
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
Policies
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Along with de-duplication, enforcement of administrative policies is a major
|
|
function of a GW.
|
|
Areas of policy include: access control, resource limits, and queuing behavior.
|
|
\end_layout
|
|
|
|
\begin_layout Subsection
|
|
Access Control
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
In order to share GWC channels, the GW will make all access control decisions.
|
|
Authentication information provided by CLI is
|
|
\series bold
|
|
never
|
|
\series default
|
|
forwarded to SRV.
|
|
Instead, the GW's own authentication information is sent to SRV.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Access control needs to be configurable on a per-PV and per-operation basis.
|
|
In similar contexts this typically takes the form of an Access Control
|
|
List, where rules are traversed in some order.
|
|
Each rule makes a decision to Allow, Deny, or Pass to the next rule.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
To allow dynamic policies, a rule may itself subscribe to several PVs and
|
|
use the values obtained, in addition to client provided information and
|
|
static configuration, to make a decision.
|
|
\end_layout
|
|
|
|
\begin_layout Subsection
|
|
Administrative Limits
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
A number of configurable administrative limits should also be provided to
|
|
limit the resource usage of potentially misbehaving clients including:
|
|
\end_layout
|
|
|
|
\begin_layout Itemize
|
|
Max # of clients for GWS
|
|
\end_layout
|
|
|
|
\begin_layout Itemize
|
|
Max # of channels per client
|
|
\end_layout
|
|
|
|
\begin_layout Itemize
|
|
Max # of concurrent operations, for each operation type
|
|
\end_layout
|
|
|
|
\begin_layout Itemize
|
|
Max # monitor queue depth
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Such limits could be made hard (fail further requests) or soft (log and
|
|
allow).
|
|
\end_layout
|
|
|
|
\begin_layout Subsection
|
|
Queuing
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
At a minimum, the default and max queue sizes should be settable by policy.
|
|
Additionally, a choice of algorithms could be provided to decide which
|
|
entries to drop when a queue overflows.
|
|
\end_layout
|
|
|
|
\end_body
|
|
\end_document
|