revise doc
This commit is contained in:
Michael Davidsaver
@ -80,7 +80,7 @@
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\begin_body
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\begin_layout Title
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PVAccess Gateway Design
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PVAccess Gateway Prototype (pva2pva)
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\end_layout
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\begin_layout Author
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@ -104,24 +104,29 @@ today
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\end_layout
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\begin_layout Section
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Scope
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Motivation
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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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In a distributed control system, there are hundreds of nodes that are part
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of the plant.
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These nodes make up the controllers and the operator consoles.
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There are many clients that may want to view the state of equipment outside
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of the control network.
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A
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\begin_inset Quotes eld
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\end_inset
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switch
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gateway
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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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allows clients outside of the control system to view the equipment while
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limiting the additional traffic on the control network.
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The gateway marshals all external requests and makes only one request into
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the control system.
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This limits the effect of external requests on the control system, thus
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maintaining the determinism required for robust operation of a facility.
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\end_layout
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\begin_layout Standard
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@ -180,7 +185,15 @@ IP Internet Protocol (v4 and/or v6)
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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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A character string
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\begin_inset Quotes eld
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\end_inset
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PV name
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\begin_inset Quotes erd
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\end_inset
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is this address.
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\end_layout
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\begin_layout Description
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@ -228,6 +241,91 @@ Gateway
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\end_layout
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\begin_layout Section
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Prototype Status
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\end_layout
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\begin_layout Itemize
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De-duplication of Channel and Monitor is implemented as described in section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:ToO"
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\end_inset
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.
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\end_layout
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\begin_layout Itemize
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The queuing algorithm in section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:queue"
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\end_inset
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is implemented and verified.
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Verified using simulated camera image.
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\end_layout
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\begin_layout Itemize
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Access Control described in section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sub:acl"
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\end_inset
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is not implemented.
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As a placeholder a global flag enables/disables write operations.
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\end_layout
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\begin_layout Section
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Test Results
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\end_layout
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\begin_layout Subsection
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High speed counter
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\end_layout
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\begin_layout Standard
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This test involves a scalar PV incrementing at a rate greater than 1 kHz.
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The goal is to stress the gateway in an attempt to provoke incorrect actions.
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Several mutex ordering bugs were discovered and corrected.
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\end_layout
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\begin_layout Subsection
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Disconnect/reconnect
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\end_layout
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\begin_layout Standard
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Restart various combinations of CLI, GW, and SRV to see that disconnection
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notices are sent to CLI and that connections are restored properly when
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all components resume running.
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\end_layout
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\begin_layout Subsection
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Image queuing contention
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\end_layout
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\begin_layout Standard
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An IOC is created with areaDetector and QSRV.
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This IOC servers a simulated image and a 10 Hz scalar counter PV.
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A comparison is made between viewing these two PVs through CA gateway and
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through pva2pva gateway.
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Through CA gateway the scalar PV is observed to
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\begin_inset Quotes eld
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\end_inset
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stutter
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\begin_inset Quotes erd
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\end_inset
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when the image update rate is as low as 1 Hz.
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Through pva2pva this is not observed.
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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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@ -241,7 +339,7 @@ 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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of sockets and data to reduce SRV resource use.
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\end_layout
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\begin_layout Paragraph
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@ -281,6 +379,13 @@ gets
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\begin_layout Section
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Theory of Operation
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:ToO"
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\end_inset
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\end_layout
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\begin_layout Subsection
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@ -455,10 +560,11 @@ is maintained by each ChannelCache entry for the GWS channels using the
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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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A periodic cleanup task runs to remove all entries with this flag cleared
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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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Each time it run, the cleanup task also clears the flag of any entry not
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removed.
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\end_layout
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\begin_layout Standard
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@ -491,10 +597,9 @@ 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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width 95col%
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height 3in
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keepAspectRatio
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@ -536,14 +641,14 @@ 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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results in the disconnection of any associated GWS Channels, and the immediate
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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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All pending operations associated with this channel are canceled.
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\end_layout
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\begin_layout Subsection
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@ -556,27 +661,38 @@ 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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Therefore, these operations pass through the GW without de-duplication.
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\end_layout
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\begin_layout Subsection
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Monitor operations
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\begin_inset CommandInset label
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LatexCommand label
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name "sub:monitor"
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\end_inset
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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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Each ChannelCache entry includes 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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Therefore, MonitorCache hits are only generated when the CLI provides a
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pvrequest which exactly matches a 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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For efficient lookup, the monitor cache is keyed using the serialized byte
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string of a pvrequest.
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\end_layout
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\begin_layout Standard
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Each MonitorCache entry keeps the most recent value received by the GWC
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which is returned immediately for new GWS subscriptions.
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\end_layout
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\begin_layout Standard
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@ -673,6 +789,13 @@ is maintained for each MonitorCacheEntry, which will be closed as soon as
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\begin_layout Section
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Channel Transmit Queuing
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:queue"
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\end_inset
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\end_layout
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\begin_layout Standard
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@ -694,10 +817,10 @@ fairness
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\end_layout
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\begin_layout Standard
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Instead of a simple FIFO fed by all Channels, each channel given a FIFO.
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The task which dequeues would do so by taking from each FIFO in turn in
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a round robin.
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This should prevent the overall latency through the queue from being dominated
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Instead of a simple FIFO fed by all Channels, each channel is give a FIFO.
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The task which dequeues does so by taking from each FIFO in turn in a round
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robin fashion.
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This prevents the overall latency through the queue from being dominated
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by one fast PV.
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\end_layout
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@ -775,17 +898,6 @@ label{line:queue3}|
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\end_layout
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\begin_layout Section
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Loop avoidance
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\end_layout
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\begin_layout Standard
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Another potential pitfall inherent in using UDP broadcasts for name resolution
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is the possibility of loops should GWS receive search requests from GWC.
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This can be avoided provided that GWS is aware of the set of endpoints
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that GWC uses to send requests, and ignores an requests from them.
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\end_layout
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\begin_layout Section
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Policies
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\end_layout
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@ -798,6 +910,13 @@ Along with de-duplication, enforcement of administrative policies is a major
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\begin_layout Subsection
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Access Control
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\begin_inset CommandInset label
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LatexCommand label
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name "sub:acl"
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\end_inset
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\end_layout
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\begin_layout Standard
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