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What is PubSub?

PubSub is a design pattern which mostly promotes a loose coupling between two kinds of entities - publishers and subscribers. Like their names suggest, in the pubsub world we have publishers who fire events, and subscribers who wish to be notified about those events when publishers push data. There might be several subscribers, several publishers, and even several channels (or nodes) where the events are sent.

Module Description

This module implements XEP-0060 (Publish-Subscribe). Due to the complexity of the protocol, the PubSub engine makes successive calls to the nodetree and node plugins in order to check the validity of requests, perform the corresponding action and return a result or appropriate error. Such an architecture makes it much easier to write custom pubsub plugins and add new storage backends. It's all about tailoring PubSub to your needs!

Options

  • iqdisc (default: one_queue)
  • host (string, default: "pubsub.@HOST@"): Subdomain for Pubsub service to reside under. @HOST@ is replaced with each served domain.
  • backend (atom, default: mnesia) - Database backend to use. mnesia and rdbms are supported currently.
  • access_create (atom, default: all): Who is allowed to create pubsub nodes.
  • max_items_node (integer, default: 10): Define the maximum number of items that can be stored in a node.
  • max_subscriptions_node (integer, default: undefined - no limitation): The maximum number of subscriptions managed by a node.
  • nodetree (binary, default: <<"tree">>): Specifies the storage and organisation of the pubsub nodes. See the section below.
  • ignore_pep_from_offline (boolean, default: true): specify whether or not we should get last published PEP items from users in our roster which are offline when we connect. The default option is true hence we will get only the last items from the online contacts.
  • last_item_cache (atom, default false): If enabled, PubSub will cache the last published items in the nodes. It may increase PubSub performance but at a price of an increased memory usage. Valid values are mnesia, rdbms and false.
  • plugins ([Plugin, ...], default: [<<"flat">>]): List of enabled pubsub plugins.
  • pep_mapping ([{Key, Value}, ...]): This permits creating a Key-Value list to define a custom node plugin on a given PEP namespace. E.g. pair {"urn:xmpp:microblog:0", "mb"} will use module node_mb instead of node_pep when the specified namespace is used.
  • default_node_config ([{Key, Value}, ...]): Overrides the default node configuration, regradless of the node plugin. Node configuration still uses the default configuration defined by the node plugin, and overrides any items by the value defined in this configurable list.
  • item_publisher (boolean, default: false): When enabled, a JID of the publisher will be saved in the item metadata. This effectively makes them an owner of this item.
  • sync_broadcast (boolean, default: false): If false, routing of notifications to subscribers is done in a separate Erlang process. As a consequence, some notifications may arrive to the subscribers in the wrong order (however, the two events would have to be published at the exact same time).

Cache Backend

Caching is disabled by default. You may enable it by specifying the backend it should use. It is not coupled with the main DB backend, so it is possible to store the cached data in mnesia, while the actual PubSub information is kept in RDBMS (and vice versa!).

Example Configuration

   {mod_pubsub, [{access_createnode, pubsub_createnode},
                 {ignore_pep_from_offline, false},
                 {backend, rdbms},
                 {last_item_cache, mnesia},
                 {max_items_node, 1000},
                 {plugins, [<<"flat">>, <<"pep">>]}
  ]},

Nodetrees

Called on get, create and delete node. Only one nodetree can be used per host and is shared by all node plugins.

<<"tree">>

Stores nodes in a tree structure. Every node name must be formatted like a UNIX path (e.g. /top/middle/leaf). When a node is created, its direct ancestor must already exist, so in order to create /top/middle/leaf, /top/middle is needed. A user may create any top-level node. A user may create a subnode of a node, only if they own it or it was created by the service.

<<"dag">>

Provides experimental support for XEP-0248 (PubSub Collection Nodes). In this case you should also add the <<"dag">> node plugin as default, for example: {plugins, [<<"dag">>,<<"flat">>,<<"hometree">>,<<"pep">>]}

Plugins

They handle affiliations, subscriptions and items and also provide default node configuration and features. PubSub clients can define which plugin to use when creating a node by adding type='plugin-name' attribute to the create stanza element. If such an attribute is not specified, the default plugin will be the first on the plugin list.

<<"flat">>

No node hierarchy. It handles the standard PubSub case.

<<"hometree">>

Uses the exact same features as the flat plugin but additionally organises nodes in a tree. Basically it follows a scheme similar to the filesystem's structure. Every user can create nodes in their own home root: e.g /home/user. Each node can contain items and/or sub-nodes.

<<"pep">>

Implementation of XEP-0060 (Personal Eventing Protocol). In this case, items are not persisted but kept in an in-memory cache. When the pep plugin is enabled, a user can have their own node (exposed as their bare jid) with a common namespace. Requires module mod_caps to be enabled.

<<"dag">>

Implementation of XEP-0248 (PubSub Collection Nodes). Every node takes a place in a collection and becomes either a collection node (and have only sub-nodes) or a leaf node (contains only items).

<<"push">>

Special node type that may be used as a target node for XEP-0357 (Push Notifications) capable services (e.g. mod_event_pusher_push). For each published notification, a hook push_notification is run. You may enable as many modules that support this hook (all module with mod_push_service_* name prefix) as you like (see for example mod_push_service_mongoosepush). This node type requires publish-options with at least device_id and service fields supplied.

Metrics

If you'd like to learn more about metrics in MongooseIM, please visit the MongooseIM metrics page.

Overall PubSub action metrics

For every PubSub action, like node creation, subscription, publication the following metrics are available:

  • count - a spiral metric showing the number of given action invocations
  • errors - a spiral metric counting the errors for a given action
  • time - a histogram metric showing the time it took to finish the action in case of success

Below there is a table describing all metrics related to PubSub actions

Name Description (when it gets incremented)
[HOST, pubsub, get, affiliations, TYPE] When node's affiliations are read
[HOST, pubsub, get, configure, TYPE] When node's configuration is read
[HOST, pubsub, get, default, TYPE] When node's defaults are read
[HOST, pubsub, get, items, TYPE] When node's items are read
[HOST, pubsub, get, options, TYPE] When node's options are read
[HOST, pubsub, get, subscriptions, TYPE] When node's subscriptions are read
[HOST, pubsub, set, affiliations, TYPE] When node's subscriptions are set
[HOST, pubsub, set, configure, TYPE] When node's configuration is set
[HOST, pubsub, set, create, TYPE] When node is created
[HOST, pubsub, set, delete, TYPE] When node is deleted
[HOST, pubsub, set, options, TYPE] When node's options are set
[HOST, pubsub, set, publish, TYPE] When an item is published
[HOST, pubsub, set, purge, TYPE] When node's items are purged
[HOST, pubsub, set, retract, TYPE] When node's items are retracted
[HOST, pubsub, set, subscribe, TYPE] When a subscriber subscribes to a node
[HOST, pubsub, set, subscriptions, TYPE] When a subscription is set (for instance accepted)
[HOST, pubsub, set, unsubscribe, TYPE] When a subscriber unsubscribes

Where:

  • HOST is the XMPP host for which mod_pubsub is running. Can be set to global if all metrics are set to be global.
  • TYPE is one of the following count, errors, time (described above the table)

Backend operations

The are also more detailed metrics measuring execution time of backend operations.

Metrics for these actions may be found under mod_pubsub_db subkey.

Backend action Description (when it gets incremented)
get_state User's state for a specific node is fetched.
get_states Node's states are fetched.
get_states_by_lus Nodes' states for user + domain are fetched.
get_states_by_bare Nodes' states for bare JID are fetched.
get_states_by_full Nodes' states for full JID are fetched.
get_own_nodes_states State data for user's nodes is fetched.
create_node A node's owner is set.
del_node All data related to a node is removed.
get_items Node's items are fetched.
get_item A specific item from a node is fetched.
add_item An item is upserted into a node.
set_item An item is updated in a node.
del_item An item is deleted from a node.
del_items Specified items are deleted from a node.
set_node A node is upserted.
find_node_by_id A node is fetched by its ID.
find_nodes_by_key Nodes are fetched by key.
find_node_by_name A node is fetched by its name.
del_node A node is deleted.
get_subnodes Subnodes of a node are fetched.
get_subnodes_tree Full tree of subnodes of a node is fetched.
get_parentnodes_tree All parents of a node are fetched.