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replisome - handsomely replicate something

Build status

The project is currently in pre-production phase. We are hacking on it.

replisome is a lightweight, flexible, and easily configurable system to export data changes from PostgreSQL. It allows a client to receive a stream of changes describing the data manipulation inside the database (INSERT, UPDATE, DELETE of records) in JSON format for all, or a specified subset of tables, with the possibility of limiting the columns and rows received.

What can you do with data changes?

  • Replication: you can apply the changes to another database and obtain a copy of the data.
  • Upgrade, downgrade: the database to which you are applying changes could be a different version.
  • Export: the database to which you are applying changes could be something else other than PostgreSQL (this is obviously only theoretical, as nobody sane would use a different database...)
  • Integrate: the thing to which you are applying the changes could be redis, memcached or some other key-value store granting fast access to data.
  • Audit, logging: would you like to write all the changes into a file?
  • Email, twitter: get notified when something important changes.
  • Whatevs: You have a stream of changes from a database, do whatever you want with it.

You may have noticed a few easy-to-brag-about buzzwords in the opening statement: here is why we feel entitled to use them:

  • Lightweight: replisome is based on PostgreSQL logical decoding, not on triggers; as such it doesn't require extra work for the database, such as inserting a record in a queue table for every record changed. This makes it more efficient than pgq and londiste.
  • Flexible: replisome allows emitting changes only on specific tables, specific columns, or even specific records. The data produced is JSON and the system doesn't care about the usage of the data: if used as a replication system the database receiving the data doesn't need to have matching tables, columns, or data types (as pglogical requires).
  • Easy to configure: the entire configuration, from the selection of the data to export to its usage, is a parameter of the script consuming the data; there is no persistent configuration (nodes, subscribers, replication sets...). Changing configuration only requires changing the configuration file the running consumer script is using, and then stopping and restarting it.

replisome is not a complete replication solution: it doesn't deal with truncate, DDL language, sequences replication, or conflicts. If you are looking for something like that take a look at pglogical instead. What it aims to be is a more flexible tool for data integration.

System description

The system is composed of two main parts:

  • The sender is a PostgreSQL logical replication decoder plugin that can be widely configured in order to choose what data to emit and how.
  • The receiver is an easy to extend Python framework allowing manipulation and consumption of data produced by a sender. It is easy to write your own extensions to this framework, or ditch it altogether and make direct use of the data produced by the sender.

Decoding plugin

The decoding plugin is the bit that lives in the PostgreSQL server used as a data source. Please refer to the documentation for an introduction to logical decoding.

Requirements

The data source must be PostgreSQL running at least version 9.4 or newer.

Build and Install

TODO: pgxn install replisome.

This thing will be packaged as an extension, have a version number, be released on PGXN... but currently it is only available on github.

The extension should be compiled and installed in a PostgreSQL installation, after which it will be available in the database clusters run by that installation.

In order to build the extension you will need a C compiler, the PostgreSQL server development packages and maybe something else that you could easily find by googling the problem.

$ git clone https://github.com/GambitResearch/replisome.git
$ cd replisome
$ make PG_CONFIG=/path/to/bin/pg_config
$ sudo make PG_CONFIG=/path/to/bin/pg_config install

The extension should be loaded in the database you want to use as data source. Currently the extension only exports a function replisome_version() which the replisome client uses to verify it is communicating with a server with known protocol: if you will use your own receiver this step is not strictly necessary:

$ psql -c "create extension replisome" "$TARGET_DATABASE"

Configuration

The cluster must be configured to use logical replication: you need to add the following parameters to postgresql.conf:

wal_level = logical
max_replication_slots = 1       # at least
max_wal_senders = 1             # at least

After changing these parameters a restart is needed.

You will also need to set permissions in pg_hba.conf to allow replication connections

local    replication     myuser                     trust
host     replication     myuser     10.1.2.3/32     trust

Every replisome consumer must connect to a replication slot, which will hold the state of the replication client (so that a stopped consumer will not miss the data: on restart it will pick up from where it left off). You can create a replication slot using:

=# select pg_create_logical_replication_slot('MY NAME', 'replisome');

The name is what will be used by the client to connect to a specific slot.

Examples

There are a few ways to obtain the changes (JSON objects) from the replisome plugin:

Examples using pg_recvlogical

You are ready to try replisome. In one terminal create a replication slot and start a replica:

$ pg_recvlogical -d postgres --slot test_slot --create-slot -P replisome
$ pg_recvlogical -d postgres --slot test_slot --start -o pretty-print=1 -f -

In another terminal connect to the database and enter some commands:

=# create table test (
   id serial primary key, data text, ts timestamptz default now());
CREATE TABLE

=# insert into test default values;
INSERT 0 1
=# insert into test (data) values ('hello');
INSERT 0 1

=# begin;
BEGIN
*=# update test set data = 'world' where id = 2;
UPDATE 1
*=# delete from test where id = 1;
DELETE 1
*=# commit;
COMMIT

The streaming connection should display a JSON description of the operations performed:

{
    "tx": [
        {
            "op": "I",
            "schema": "public",
            "table": "test",
            "colnames": ["id", "data", "ts"],
            "coltypes": ["int4", "text", "timestamptz"],
            "values": [1, null, "2017-05-13 13:15:28.052318+01"]
        }
    ]
}
{
    "tx": [
        {
            "op": "I",
            "schema": "public",
            "table": "test",
            "values": [2, "hello", "2017-05-13 13:15:35.140594+01"]
        }
    ]
}
{
    "tx": [
        {
            "op": "U",
            "schema": "public",
            "table": "test",
            "values": [2, "world", "2017-05-13 13:15:35.140594+01"],
            "keynames": ["id"],
            "keytypes": ["int4"],
            "oldkey": [2]
        }
        ,{
            "op": "D",
            "schema": "public",
            "table": "test",
            "oldkey": [1]
        }
    ]
}

Options

The plugin output content and format is configured by several options passed to the START_REPLICATION command (e.g. using the -o option of pg_recvlogical, the psycopg start_replication() method etc).

pretty-print [bool] (default: false)
Add whitespace to the output for readibility.
include [json]

Choose which tables and filter content from those tables. This command together with exclude can be used several times: each table will be considered for inclusion or exclusion by matching it against all the commands specified in order from top to bottom. The last matching command will override previous commands. (e.g. you may exclude an entire schema and then include only one specific table from it).

The parameter is a JSON object which may contain the following keys:

  • table: match a table with this name, in any schema
  • tables: match all the tables whose name matches a regular expression, in any schema
  • schema: match all the tables in a schema
  • schemas: match all the tables in all the schemas whose name matches a regular expression

These keys will establish if a table matches the configuration object. At least one schema or a table must be specified. The following options can be specified too, and they will affect any table included:

  • columns: only emit the columns specified (as a JSON array)
  • skip_columns: don't emit the columns specified (as a JSON array)
  • where: only emit the row matching the condition specified as an SQL expression matching the table columns, like in a CHECK clause.

Example (as pg_recvlogical option):

-o '{"tables": "^test.*", "skip_columns": ["ts", "wat"], "where": "id % 2 = 0"}'
exclude [json]
Choose which tables to exclude. The format is the same as include but only the tables/schemas can be specified, no rows or columns.
include-xids [bool] (default: false)

If true, include the id of each transaction:

{
    "xid": 5360,
    "tx": [
        {   ...
include-lsn [bool] (default: false)

Include the Log Sequence Number of the transaction:

{
    "nextlsn": "0/3784C40",
    "tx": [
        {   ...
include-timestamp [bool] (default: false)

Include the commit time of the transaction:

{
    "timestamp": "2017-05-13 03:19:29.828474+01",
    "tx": [
        {   ...
include-schemas [bool] (default: true)
Include the schema name of the tables.
include-types [bool] (default: true)
Include the types of the table columns.
include-empty-xacts [bool] (default: false)
If true, send information about transactions not containing data changes (e.g. ones only performing DDL statements). Only the metadata (e.g. time, txid) of the transaction are sent.
write-in-chunks [bool] (default: false)
If true, data may be sent in several chunks instead of a single message for the entire transaction. Please note that a single chunk may not be a valid JSON document and the client is responsible for aggregation of received parts.

Consumer Framework

The consumer framework consists of a script entry point called replisome, taking a configuration file to describe where to read the data, how to transform it and what to do with it. Any Python callable can be used to transform and consume data. A few useful objects are provided as part of the package.

Requirements

Python 2.7 or later

Installation

TODO: pip install replisome

Currently, clone the repos and run python setup.py install

Usage

The replisome command line parameters are:

usage: replisome [-h] [--dsn DSN] [--slot SLOT] [-v | -q] [configfile]

Receive data from a database, and do something with it.

positional arguments:
  configfile     configuration file to parse; if not specified print to
                 stderr

optional arguments:
  -h, --help     show this help message and exit
  --version      show program's version number and exit
  --dsn DSN      database to read from (overrides the config file)
  --slot SLOT    the replication slot to connect to (overrides the config
                 file)
  -v, --verbose  print debugging information to stderr
  -q, --quiet    minimal output on stderr

If configfile is not specified, --dsn and --slot must be used: the script will print on stdout all the changes read in the database connected. More interesting stuff can be done specifying a configfile.

Configuration

The replisome configuration file must be a YAML file describing a process pipeline (one receiver, zero or more filters, one consumer). Example:

receiver:
    class: JsonReceiver
    dsn: "dbname=source host=sourcedb"
    slot: someslot
    options:
        pretty_print: false
        includes:
          - schema: myapp
            tables: '^contract(_expired_\d{6})?$'
            where: "seller in ('alice', 'bob')"
          - schema: myapp
            table: account
            skip_columns: [password]

filters:
  - class: TableRenamer
    options:
        from_schema: myapp
        to_schema: otherapp

consumer:
    class: DataUpdater
    options:
        dsn: "dbname=target host=targetdb"
        skip_missing_columns: true

Every object is specified by a class entry, which should be the name of one of the objects provided by the package or a fully qualified Python callable (e.g. mypackage.mymodule.MyClass). In either case the object will be called passing the contents of the options object as keyword arguments.

Receivers must subclass the TODO class; filters and consumers can be any callable object (i.e. the object returned by the class specified in the configuration file must be a callable itself): filters will take a JSON message as input (decoded as Python objects) and should return a new message, which will be passed to the following filters and eventually to the consumer. If a filter returns None the message is dropped. The consumer must be a callable taking a message too. The return value is discarded.

Only after the consumer has processed a message will the server receive a notification that the message has been consumed. If processing is interrupted for any reason (e.g. user interruption, network error, Python exception), then replication will restart from the point where it was interrupted.

License

Copyright (c) 2013-2017, Euler Taveira de Oliveira
Copyright (c) 2017, Gambit Research Ltd.
All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of Gambit Research Ltd. nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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