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<h2><a href="https://wiki.asterisk.org/wiki/display/AST/Stasis+Core+Specification">Stasis Core Specification</a></h2>
<h4>Page <b>added</b> by <a href="https://wiki.asterisk.org/wiki/display/~mjordan">Matt Jordan</a>
</h4>
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<h1><a name="StasisCoreSpecification-StasisCore"></a>Stasis Core</h1>
<h2><a name="StasisCoreSpecification-Introduction"></a>Introduction</h2>
<p>Asterisk has used a publish/subscribe event subsystem for device state and MWI notifications for some time. This model has served Asterisk well: it easily allows consumers of MWI and device state to process those events without impacting the producers of those notifications. Ideally, AMI - and other interfaces that consume Asterisk state - would use such a mechanism to interact with the Asterisk core and modules. Unfortunately, channel state is not available in such a mechanism, nor is there a generic way to publish a wide variety of messages.</p>
<p>Stasis Core fulfills that role. It unifies the current AMI events, channel state, along with the existing device state/MWI to create a generic publish/subscribe message bus. This allows:</p>
<ul>
<li>AMI to be built on top of the message, isolating changes in the AMI protocol from the Asterisk core and vice versa</li>
<li>New interfaces to be developed rapidly, as they can share the information pipeline that feeds AMI and other existing components</li>
<li>Dramatically simplifies components that have to track channel state, as they no longer have to be distributed throughout the Asterisk codebase</li>
</ul>
<div class='panelMacro'><table class='noteMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/warning.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td><b>Disclaimer</b><br />There's a bit of a chicken and the egg problem in the design/implementation right now - do we refactor the existing event system to be Stasis Core? Or is Stasis Core simply an expanded event system that includes channel state and arbitrary keyed messages?
<p>Rather than solving this particular problem, this specification is attempting to focus on the contracts that users of Stasis Core (regardless of how it gets constructed) need to know about. As such, the API will look <b>awfully</b> similar to the existing event system. In the end, it may end up <b>being</b> the existing event system, and the names will stay the same. View this as contracts the API has to provide, rather than a guarantee that it will end up being exactly what is specified below.</p>
<p>(However, by the end of all of this, this should reflect the actual API)</p></td></tr></table></div>
<h2><a name="StasisCoreSpecification-Terminology"></a>Terminology</h2>
<div class='table-wrap'>
<table class='confluenceTable'><tbody>
<tr>
<th class='confluenceTh'> Term </th>
<th class='confluenceTh'> Definition </th>
</tr>
<tr>
<td class='confluenceTd'> AMI </td>
<td class='confluenceTd'> The Asterisk Manager Interface, a legacy interface that is built on top of Stasis Core </td>
</tr>
<tr>
<td class='confluenceTd'> Stasis HTTP </td>
<td class='confluenceTd'> An interface built on top of Stasis Core that exposes functionality in a REST API </td>
</tr>
</tbody></table>
</div>
<h2><a name="StasisCoreSpecification-ProtocolOverview"></a>Protocol Overview</h2>
<p>The Stasis Core is a binary message bus that allows components to publish messages to interested subscribers. Messages are comprised of key/type/value tuples. The type can be anything as simple as an <tt>int</tt> to as complex as a struct resident in heap memory. It is up to the subscribers of messages to understand how to decode the various typed values and - if they do not know how to decode the values - to pass on them. Stasis Core provides:</p>
<ul>
<li>A thread-safe mechanism for publishing messages to subscribers, such that publisher threads are never blocked by message delivery</li>
<li>A cache for objects that allows said objects to be updated and queried by subscribers</li>
</ul>
<h2><a name="StasisCoreSpecification-SemanticsandSyntax"></a>Semantics and Syntax</h2>
<h3><a name="StasisCoreSpecification-MessageSending%2FReceiving"></a>Message Sending/Receiving</h3>
<p>Any Asterisk module may publish messages on the Stasis Core message bus. Messages are identified by a MessageID field. Subscribers subscribe for messages based on the MessageID field, and register a callback handler that will be called by Stasis Core when a message matching the MessageID field is detected.</p>
<h3><a name="StasisCoreSpecification-Threading%2FMemoryModel"></a>Threading/Memory Model</h3>
<p>As the Stasis Core exists as a service available to modules in Asterisk, it is important to define ownership of message objects and what threads they are serviced on.</p>
<p>Messages and their data places on the Stasis Core message bus are immutable; that is, once a value is formatted and placed into a message, it cannot be changed. This means that values MUST not be reference counted objects and that publishers MUST not keep a reference to any value placed in a message. Similarly, subscribers MUST not store a reference to values placed on a message; once a message is delivered and processed Stasis Core is responsible for reclaiming the memory associated with any values.</p>
<h4><a name="StasisCoreSpecification-OwnershipduringPublication"></a>Ownership during Publication</h4>
<p>When a publisher places a Stasis message on the bus, it relinquishes ownership of the message object and must not hold any references to the message or values contained within the message. Placing a message on a queue for servicing by the bus occurs within the context of the thread doing the publishing.</p>
<p>When a subscriber receives a published message, it is done in the context of a thread that exists on a thread pool. It is up to the subscriber to determine if they want to service the message completely on the thread pool thread, or marshal the data contained in the message onto some other thread. When the defined handler entry point returns, the thread servicing the request is returned to the pool. The message that was handled is implicitly reclaimed when the routine returns; if the message elements need to persist beyond the lifetime of the message handling, the subscriber must copy the message data.</p>
<h4><a name="StasisCoreSpecification-OwnershipduringSubscription"></a>Ownership during Subscription</h4>
<p>Subscribing for messages happens within the context of the thread performing the subscribing. A subscription token is handed to the subscribing module that represents that particular subscription session. A subscription may be cancelled through the token given to the subscriber.</p>
<div class='panelMacro'><table class='noteMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/warning.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td>Once a subscriber has cancelled a subscription, published events may still be in flight to that subscriber. While no new events may be placed in the subscriber's queue, they should be ready to handle events that are currently in flight.</td></tr></table></div>
<h3><a name="StasisCoreSpecification-MessageLayout"></a>Message Layout</h3>
<p>Messages placed on the Stasis Core bus are variable length and may contain any number of fields. Each message is comprised of a header, and a payload. The header provides an identifier for the message, publisher information, and other data to be consumed by subscribers to determine if the message is something they want to process. The payload contains information that is self describing and can be extracted by subscribers.</p>
<div class='panelMacro'><table class='noteMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/warning.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td>Multiple modules may publish messages with the same MessageID field, in which case the messages are treated as being identical. If the contents of the messages are different, it is the job of subscribers to parse out the key/type/value tuples and handle them appropriately.</td></tr></table></div>
<p>For the purposes of this specification, we leave the actual message definitions to the implementation and instead focus on the specification of the Stasis message headers.</p>
<h4><a name="StasisCoreSpecification-HeaderDefinition"></a>Header Definition</h4>
<div class='table-wrap'>
<table class='confluenceTable'><tbody>
<tr>
<th class='confluenceTh'> Field </th>
<th class='confluenceTh'> Type </th>
<th class='confluenceTh'> Required </th>
<th class='confluenceTh'> Description </th>
</tr>
<tr>
<td class='confluenceTd'> ID </td>
<td class='confluenceTd'> char * </td>
<td class='confluenceTd'> Yes </td>
<td class='confluenceTd'> A unique identifier for the message on the bus. Note that this doesn't have to be a UUID, as it only has to be unique on this instance of Asterisk. A scheme that allows for globally unique identifiers across time is sufficient. </td>
</tr>
<tr>
<td class='confluenceTd'> ResponseID </td>
<td class='confluenceTd'> char * </td>
<td class='confluenceTd'> No (NULL) </td>
<td class='confluenceTd'> If this message is a response to another message placed on the Stasis bus, the identifier of the message it is a response for. </td>
</tr>
<tr>
<td class='confluenceTd'> MessageID </td>
<td class='confluenceTd'> ast_stasis_message_type </td>
<td class='confluenceTd'> Yes </td>
<td class='confluenceTd'> The unique identifier for the message. Determined by the publisher. </td>
</tr>
<tr>
<td class='confluenceTd'> Timestamp </td>
<td class='confluenceTd'> long </td>
<td class='confluenceTd'> Yes </td>
<td class='confluenceTd'> An epoch timestamp for the message. This is provided by Stasis Core when the messages are handled. </td>
</tr>
</tbody></table>
</div>
<h4><a name="StasisCoreSpecification-Key%2FType%2FValueTuples"></a>Key/Type/Value Tuples</h4>
<p>Each value in a message is defined by a key/type/value tuple. The key is a <tt>char *</tt> that uniquely identifies for a given message the value in that message. The type is an enumeration value that defines the type the value has. The value is some blob of data to be extracted based on the type.</p>
<h3><a name="StasisCoreSpecification-API"></a>API</h3>
<div class='panelMacro'><table class='warningMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/forbidden.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td>If any section was up for significant changes, it'd be this one. Everything here is subject to change still.</td></tr></table></div>
<h4><a name="StasisCoreSpecification-Types"></a>Types</h4>
<h5><a name="StasisCoreSpecification-%7B%7Baststasissub%7D%7D"></a><tt>ast_stasis_sub</tt></h5>
<p>A subscription token returned from successful subscriptions.</p>
<h5><a name="StasisCoreSpecification-%7B%7Bastnvobject%7D%7D"></a><tt>ast_nv_object</tt></h5>
<p>A tuple containing name, a data type, and a value matching that data type.</p>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessagetype%7D%7D"></a><tt>ast_stasis_message_type</tt></h5>
<p>An enumeration containing high level message types. Currently, these would be anticipated to encompass:</p>
<ul>
<li>Mailbox State</li>
<li>Device State</li>
<li>Channel State</li>
<li>Bridge State</li>
<li>Presence</li>
</ul>
<p>Note that these do <b>not</b> map to AMI events. Rather, they are high level concepts in Asterisk that a large variety of messages could correspond to.</p>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessage%7D%7D"></a><tt>ast_stasis_message</tt></h5>
<p>An object containing the generic message information.</p>
<h4><a name="StasisCoreSpecification-Subscription"></a>Subscription</h4>
<h5><a name="StasisCoreSpecification-%7B%7Baststasissubscribe%7D%7D"></a><tt>ast_stasis_subscribe</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Subscribe to stasis messages
*
* \param message_type The type of messages to subscribe to
* \param cb The function to be called with messages
* \param description Description of the subscription.
* \param userdata data to be passed to the event callback
*
* The rest of the arguments to this function specify additional parameters for
* the subscription to filter which messages are passed to this subscriber. The
* arguments must be in sets of ast_nv_object, which define a name/type/value
* tuple.
* \code
* <struct ast_nv_object *>, [ struct ast_nv_object * ], SENTINEL
* \endcode
* and must end with SENTINEL.
*
* \return This returns a reference to the subscription for use with
* un-subscribing later. If there is a failure in creating the
* subscription, NULL will be returned.
*
*/
struct ast_stasis_sub *ast_stasis_subscribe(
ast_stasis_message_type message_type,
ast_stasis_cb_t callback,
void *userdata,
...);</pre>
</div></div>
<h5><a name="StasisCoreSpecification-%7B%7Baststasisunsubscribe%7D%7D"></a><tt>ast_stasis_unsubscribe</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Un-subscribe from stasis messages
*
* \param message_sub This is the reference to the subscription returned by
* ast_stasis_subscribe.
*
* This function will remove a subscription and free the associated data
* structures.
*
* \return NULL for convenience.
*/
struct ast_stasis_sub *ast_stasis_unsubscribe(struct ast_stasis_sub *message_sub);</pre>
</div></div>
<h4><a name="StasisCoreSpecification-Events"></a>Events</h4>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessagenew%7D%7D"></a><tt>ast_stasis_message_new</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Create a new stasis message
*
* \param message_type The type of message to create
*
* The rest of the arguments to this function specify information to add to the
* message. These must be ast_nv_objects.
* \code
* <struct ast_nv_object*>, [ struct ast_nv_object * ], SENTINEL
* \endcode
* and must end with SENTINEL.
*
* \return This returns the message that has been created. If there is an error
* creating the event, NULL will be returned.
*
* Example usage:
*
* \code
* if (!(event = ast_stasis_message_new(AST_EVENT_CHANNEL,
* ast_create_nv_object_from_channel("channel_one", chan1),
* ast_create_nv_object_from_channel("channel_two", chan2),
* SENTINEL))) {
* return;
* }
* \endcode
*
*/
struct ast_stasis_message *ast_stasis_message_new(enum ast_stasis_message_type message_type, ...);</pre>
</div></div>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessagenewresponse%7D%7D"></a><tt>ast_stasis_message_new_response</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Create a new stasis message that responds to a previous message
*
* \param original The original message to respond to
* \param message_type The type of message to create
*
* The rest of the arguments to this function specify information to add to the
* message. These must be ast_nv_objects.
* \code
* <struct ast_nv_object*>, [ struct ast_nv_object * ], SENTINEL
* \endcode
* and must end with SENTINEL.
*
* \return This returns the message that has been created. If there is an error
* creating the event, NULL will be returned.
*
* Example usage:
*
* \code
* if (!(event = ast_stasis_message_new_response(original, AST_EVENT_CHANNEL,
* ast_create_nv_object_from_channel("channel_one", chan1),
* ast_create_nv_object_from_channel("channel_two", chan2),
* SENTINEL))) {
* return;
* }
* \endcode
*
*/
struct ast_stasis_message *ast_stasis_message_new_response(struct ast_stasis_message *original, enum ast_stasis_message_type message_type, ...);</pre>
</div></div>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessagedestroy%7D%7D"></a><tt>ast_stasis_message_destroy</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Destroy a stasis message
*
* \param message the message to destroy
*
* \return Nothing
*
* \note Messages that have been queued should *not* be destroyed by the code that
* created the message. It will be automatically destroyed after being
* dispatched to the appropriate subscribers.
*
* \returns NULL as a convenience
*/
struct ast_stasis_message * ast_stasis_message_destroy(struct ast_stasis_message *message);</pre>
</div></div>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessagequeue%7D%7D"></a><tt>ast_stasis_message_queue</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Queue a stasis message
*
* \param message the message to be queued
*
* \retval zero success
* \retval non-zero failure. Note that the caller of this function is
* responsible for destroying the event in the case of a failure.
*
* This function queues a message to be dispatched to all of the appropriate
* subscribers. This function will not block while the message is being
* dispatched because the message is queued up for a dispatching thread
* to handle.
*/
int ast_stasis_message_queue(struct ast_stasis_message *message);</pre>
</div></div>
<h5><a name="StasisCoreSpecification-%7B%7Baststasismessagequeueandcache%7D%7D"></a><tt>ast_stasis_message_queue_and_cache</tt></h5>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">/*!
* \brief Queue and cache a message
*
* \param message the message to be queued and cached
*
* \details
* The purpose of caching messages is so that the core can retain the last known
* information for messages that represent some sort of state. That way, when
* code needs to find out the current state, it can query the cache.
*
* \retval 0 success
* \retval non-zero failure.
*/
int ast_stasis_message_queue_and_cache(struct ast_stasis_message *message);</pre>
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