# CPG ([CloudI]( Process Groups)

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## Purpose

cpg provides a process group interface that is focused on
availability and partition tolerance (in the CAP theorem).
The pg process group implementation added in Erlang/OTP 23 by
WhatsApp Inc. (Meta Platforms Inc. / Facebook Inc.) is based on cpg.
The cpg interface is compatible with pg2 (removed in Erlang/OTP 24).

## Features (Compare and Contrast)

### cpg

* By default, cpg utilizes Erlang strings for group names (list of integers) and provides the ability to set a pattern string as a group name.  A pattern string is a string that includes the `"*"` or `"?"` wildcard characters (equivalent to a ".+" regex while `"**"`, `"??"`, `"*?"`, and `"?*"` are forbidden).  When a group name is a pattern string, a process can be retrieved by matching the pattern (more information at the [CloudI FAQ](  To not use this approach for group names, refer to the [Usage](#usage) section below.
* cpg provides its internal state for usage in separate Erlang processes as cached data with the `cpg_data` module.  That approach is more efficient than usage of ets.
* Each cpg scope is an atom used as a locally registered process name for the cpg scope Erlang process.  Separate cpg scopes may be used to keep group memberships entirely separate.
* cpg data lookups are done based on the Erlang process being local or remote, or the relative age of the local membership to the group, or with random selection (using the terminology `closest`, `furthest`, `random`, `local`, `remote`, `oldest`, `newest`).  `closest` prefers local processes if they are present while `furthest` prefers remote processes if they are present.  The `oldest` process in a group is naturally the most stable process.
* cpg provides an interface for `via` process registry use (examples are provided in the [tests](
* cpg [supports](#usage) hidden node connections (hidden node connections are a way to avoid distributed Erlang node connection limitations by not creating a fully-connected network topology).

### pg (>= Erlang/OTP 23) (

* pg uses one monitor per remote node (it takes longer to update a group after an Erlang process dies and may never remove remote group members).
* pg uses ets while cpg does not (cpg instead provides cached data for more efficient access to the process group data).

### pg2 (< Erlang/OTP 24)

* pg2 uses global:trans/2 which is unable to handle network or node failures.
* pg2 uses ets while cpg does not (cpg instead provides cached data for more efficient access to the process group data).

### [gproc]( / [syn](

* Both are focused on consistency with leader election and are unable to be available when suffering network or node failures.  Failures can cause unpredictable conflict resolution, in an attempt to achieve consistency.

## Design

cpg is a Commutative/Convergent Replicated Data-Type (CRDT) that uses
node ownership of Erlang processes to ensure a set of keys has
add and remove operations that commute with an internal map data structure.
The cpg module provides add and remove operations with the function names
join and leave, that may only be called on the node that owns the
Erlang process which is the value for the join or leave operation.
The key is the process group name which represents a list of Erlang processes
(with an single Erlang process being able to be added or removed any
number of times).

All cpg join and leave operations change global state as a
Commutative Replicated Data-Type (CmRDT) by sending the operation to the
associated cpg Erlang process as a distributed Erlang message to all remote
nodes after the operation successfully completes on the local node.

cpg also uses distributed Erlang node monitoring to handle netsplits as a
Convergent Replicated Data-Type (CvRDT) by sending all of the internal
cpg state to remote nodes that have recently connected.  The associated
cpg Erlang process on the remote node then performs a merge operation to
make sure the count of each Erlang pid is consistent with the internal
cpg state it received.

The CRDT functionality in cpg is most similar to the
[POLog (Partially Ordered Log of operations)](#references)
though the cpg approach would instead be called an
"Ordered Log of operations" because it is depending on Erlang messaging on
a local node to have causal ordering (no vclocks are necessary to establish
causality on the local node with the cpg scope Erlang process message queue
providing an "Ordered Log of operations").  After a cpg operation
completes successfully on the local node, it is sent to all remote nodes which
act as read-only views of the local node.

The cpg scope process on the local node enforces causality by existing as the
only read/write store of the local process memberships (i.e., serialized
mutability similar to a mutex lock) while the remote nodes obtain the
process memberships as soon as possible.  If a remote node is down due to a
netsplit, it will obtain the local node's state once it reconnects as
described above.

## Build

    rebar get-deps
    rebar compile

## Usage

If you need non-string (not a list of integers) group names,
set the cpg application `group_storage` env value to a module name that
provides a dict module interface
(e.g., use `dict` or [`mapsd`](

Node names that have a prefix of `NODE_script_process`
(where NODE is the current node name) are automatically ignored
because they are assumed to be release scripts (e.g., nodetool).
To process hidden node membership data, set the cpg application `node_type`
env value to `all` (instead of `visible`).

## Example

    $ erl -sname cpg@localhost -pz ebin/ -pz deps/*/ebin/

    (cpg@localhost)1> reltool_util:application_start(cpg).
    (cpg@localhost)2> cpg:join(groups_scope1, "Hello", self()).
    (cpg@localhost)3> cpg:join(groups_scope1, "World!", self()).
    (cpg@localhost)4> cpg:get_local_members(groups_scope1, "Hello").
    (cpg@localhost)5> cpg:get_local_members(groups_scope1, "World!").
    (cpg@localhost)6> cpg:which_groups(groups_scope1).
    (cpg@localhost)7> cpg:which_groups(groups_scope2).

What does this example mean?  The cpg interface allows you to define groups of
Erlang processes and each group exists within a scope.  A scope is represented
as an atom which is used to locally register a cpg Erlang process using
`start_link/1`.  For a given cpg scope, any Erlang process can join or leave
a group.  The group name is a string (list of integers) due to the default
usage of the trie data structure, but that can be changed
(see the [Usage](#usage) section above).  If the scope is not specified, the default
scope is used: `cpg_default_scope`.

In the example, both the process group "Hello" and the process group "World!"
are created within the `groups_scope1` scope.  Within both progress groups,
a single Erlang process is added once.  If more scopes were required, they
could be created automatically by being provided within the cpg application
scope list.  There is no restriction on the number of process groups that
can be created within a scope, and there is nothing limiting the number
of Erlang processes that can be added to a single group.  A single Erlang
process can be added to a single process group in a single scope multiple times
to change the probability of returning a particular Erlang process, when
only a single process is requested from the cpg interface (e.g., from
the `get_closest_pid` function).

## Tests

    rebar get-deps
    rebar compile
    ERL_LIBS="/path/to/proper" rebar eunit

## Author

Michael Truog (mjtruog at protonmail dot com)

## License

MIT License

## References

1. Carlos Baquero, Paulo Sérgio Almeida, Ali Shoker.  Making operation-based crdts operation-based. In Proceedings of the First Workshop on Principles and Practice of Eventual Consistency, page 7. ACM, 2014. [](
1. Carlos Baquero, Paulo Sérgio Almeida, Ali Shoker.  Pure Operation-Based Replicated Data Types. 2017. [](