MySQL binlogs give maxwell access to the raw bytes for each update that happens. In order to generate useful output, Maxwell needs to know the data type of every column, so that it can interpret the bytes as a number, string, boolean, etc.
There is a set of "base schema" tables in the maxwell database -
databases. This is where we capture the initial schema, the first time Maxwell is run.
As time progresses, maxwell will see any modifications you make to the schema (these are part of the binlog). As each change occurs, Maxwell will generate an internal representation of what changed. Maxwell stores these diffs in the
schemas table - each diff contains the following information to place it in the timeline of your database:
gtid_set): the exact point in the binlog stream where the schema change occurred
deltas: the internal representation of the schema change
base_schema_id: the previous schema that this delta applies to
last_heartbeat_read: the most recent maxwell heartbeat seen in the binlog prior to this change
server_id: the server which this schema applies to
This information creates a concrete timeline of the history of your schema for a given server. Given any binlog file+position (or gtid) and a last_heartbeat value, the current schema can be found by finding the "most recent" schema for this server_id, then following the chain of
base_schema_id until it terminates (in a
null, which means we've reached the initial captured schema).
"Most recent" should be fairly intuitive - firstly we sort by
last_heartbeat_read, then by
binlog_position. We limit the search to values "before" the binlog position we're searching for, because we don't want to use a schema corresponding to a change that is "in the future" - i.e. further ahead in the binlog than our current position.
Schemas are important for master failover. When Maxwell detects that it is talking to a new
server_id (one that differs from its stored
position), it attempts a master failover (if enabled). It searches backwards in the new servers binlog files, looking for an update to
maxwell.heartbeats corresponding to the timestamp stored in its position table.
Once it finds this (unique) update, it knows the binlog location for both the old and new master which correspond to the exact same event.
Using this information, maxwell creates a merge point - it finds the active schema for the old master's stored position, and then creates a new schema entry with an empty delta, the new
server_id, and a base_schema_id of the previous schema. In this way, Maxwell is able to create a chain of schema updates even across different servers with different sets of binlogs.