pvxs/documentation/client.rst

Client

pvxs::client::Context represents a PVA protocol client. :

#include <pvxs/client.h>
namespace pvxs { namespace client { ... } }

Configuration

The recommended starting point is creating new context configured from $PVA_* environment variables. Use pvxs::server::Config::from_env and then pvxs::server::Config::build.

EPICS_PVA_ADDR_LIST

A list of destination addresses to which UDP search messages will be sent. May contain unicast and/or broadcast addresses.

EPICS_PVA_AUTO_ADDR_LIST

If "YES" then all local broadcast addresses will be implicitly appended to $EPICS_PVA_ADDR_LIST "YES" if unset.

EPICS_PVA_BROADCAST_PORT

Default UDP port to which UDP searches will be sent. 5076 if unset.

using namespace pvxs;
// Context configured from process environment
client::Context ctxt = client::Config::from_env().build();

Programatic configuration can be accomplished by explicitly filling in a pvxs::server::Config.

Making Requests

A pvxs::client::Context instance is the entry point for all client network operations. Begin by calling one of the info(), get(), put(), rpc(), or monitor() methods. Each of these methods returns a *Builder object which can be used to provide additional configuration in what are in effected named arguments.

pvxs::client::Context

Get/Info

pvxs::client::Context::info and pvxs::client::Context::get return a pvxs::client::GetBuilder to prepare either a get() or info() (GET_FIELD) operation. The practical difference being that info() yields a Value which will never have any fields marked.

pvxs::client::GetBuilder

Put

pvxs::client::Context::put returns a pvxs::client::PutBuilder to prepare a put() operation. In the generic form of put(), the field values to sent have to be passed to the builder callback. This is necessary as the server mandated PV type definition is not known when an Put operation is initiated.

Additionally, a put operation will by default first fetch the present value of the PV and provide it to the builder callback. This allows eg. to perform string to index lookup when writing to an NTEnum.

pvxs::client::PutBuilder

RPC

pvxs::client::Context::rpc returns a pvxs::client::RPCBuilder to prepare an rpc() operation. rpc() differs from put() in that the call determines the type definition by providing a Value directly, so no builder callback is needed.

pvxs::client::RPCBuilder

Operation and Result

The exec() method of the *Builder objects returns a shared_ptr to an pvxs::client::Operation handle, which represents the in-progress network operation. The caller must retain this handle until completion, or the operation will be implicitly cancelled.

When an Operation completes, a pvxs::client::Result is passed to the result() callback. This object holds either a pvxs::Value if the operation succeeded, or an exception.

pvxs::client::Operation

pvxs::client::Result

Monitor

pvxs::client::Context::monitor returns a pvxs::client::MonitorBuilder to prepare a MONITOR operation. The result of this preparation is a pvxs::client::Subscription which represents the in-progress network operation. The caller must retain this handle or the operation will be implicitly cancelled.

Until cancelled, a Subscription will attempt to (re)connect to the requested PV.

A Subscription object allows access to an queue of data updates as Value and events/errors as exceptions. The pvxs::client::Subscription::pop method will remove an entry from the queue, or return an empty/invalid Value. Data updates are returned as a valid Value. Events/errors are thrown as exceptions.

The special exceptions pvxs::client::Connected, pvxs::client::Disconnect, and pvxs::client::Finished have specific meaning for a Subscription.

Connected

Depending on pvxs::client::MonitorBuilder::maskConnected (default true). Queued when a Subscription becomes connected. The Connected object include the server host:port as well as a (client) time of connection.

Disconnect

Depending on pvxs::client::MonitorBuilder::maskDisconnected (default false). Queued when a Subscription becomes disconnected.

Finished

Depending on pvxs::client::MonitorBuilder::maskDisconnected (default false). Queued when a the server indicates that Subscription will receive no more date updates as a normal completion. Finished is a sub-class of Disconnect.

There are several aspects of a Subscription which may be selected through the MonitorBuilder. The special pvxs::client::Connected and pvxs::client::Disconnect "errors" may appear in the event queue

pvxs::client::MonitorBuilder

pvxs::client::Subscription

Threading

A client Context will invoke user callback functions from one or more internal worker threads. However, it is guaranteed that callbacks relating to a given Channel (PV name + priority) will never be executed concurrently. This implies that callbacks for a single operation will also never be executed concurrently.

User code must avoid doing unnecessary work from within a callback function as this will prevent other callbacks from be executed.

Ownership

User provided callbacks are in the form of std::function which may, directly or indirectly, store shared_ptr<> instances. The returned Operation and Subscription instances may be treated as storing the std::function instance(s) and thus any shared_ptr<> captured in them.

Therefore, in order to avoid a resource leak, it is advisable to consider whether a returned Operation or Subscription may participate in a reference loop.

For example, the following creates a reference loop between the Operation instance and the "mystruct" instance.

struct mystruct {
    std::shared_ptr<Operation> op; // <-- Danger!
};
auto myptr = std::make_shared<mystruct>();

Context ctxt(...);
myptr->op = ctxt.get("pv:name")
                .result([ctxt](Result&& result) {
                })
                .exec();

While such loops can be explicitly broken (eg. by NULLing 'myptr->op') it is strongly recommended to avoid such situations as unexpected (exceptional) conditions can easily lead to resource leaks which are quite difficult to detect and isolate.

Where possible it is recommended to capture weak_ptr<> instances.

pvRequest

All operations except info() (GET_FIELD) take a Value which servers may use to modify or qualify the operation. Conventionally, the two ways this may be done is to provide a mask to limit the (sub)fields for which data is returned. Secondly, to provide certain well-known options to modify the operation.

the pvRequest conditions may be specified in three ways through the methods of pvxs::client::detail::CommonBuilder exposed through the individual *Builder types.

Programatic

The field() and record() methods.

Textual

The pvRequest() method accepts a string which is parsed into calls to the field() and record() methods. These two approaches may be intermixed.

Fallback

The rawRequest() method accepts an externally assembled Value which is sent without modification.

pvxs::client::detail::CommonBuilder

Syntax

The parser byhind pvxs::client::detail::CommonBuilder::pvRequest understands the following grammar.

pvRequest : | entry | pvRequest entry entry : field | record | field_name field : "field" "(" field_list ")" record : "record" "[" option_list "]" field_list : | field_name | field_list "," field_name option_list : | option | option_list option option : key "=" value

For examples:

• "field()"
• "field(value)"
• "value"
• "field(value,alarm)"
• "field(value)field(alarm)"
• "record[wait=true]"
• "field()record[wait=true]"
• "field(value)record[wait=true]"

Misc

pvxs::client::Config

pvxs::client::Connected

pvxs::client::Disconnect

pvxs::client::Finished

pvxs::client::RemoteError