External Slots

Wallaby provisions one publication and one logical replication slot for its own capture. If the same database also feeds a separate CDC consumer - an ELT or replication tool such as Airbyte, Debezium, or Fivetran running in pgoutput mode - that consumer needs its own publication + slot, scoped to its own tables.

AddExternalSlot lets Wallaby create and maintain those for you, so the slot is provisioned (and kept in sync) as part of your normal deployment instead of requiring seperate management. Wallaby never consumes from an external slot, it just manages it for you.

Declare a slot

builder.Services.AddWallaby<AppDbContext>(cdc =>
{
    cdc.UseConnectionString(conn)
       .AddMeilisearchSink("meili", m => { /* ... */ })
       .Map<Product>().ToSink("meili", "products").UsingTransform(/* ... */)

       // Provision a publication + slot for an external ELT tool.
       .AddExternalSlot("elt", s => s
            .WithPublication("elt_pub")          // optional; defaults to "elt_pub" (= "{slot}_pub")
            .ForTable("public", "orders")        // by schema-qualified name
            .ForTable("customers")               // schema defaults to "public"
            .ForEntity<Product>());              // or by EF entity type (resolved to its table)
});

Point your external tool at the slot name (elt) and publication name (elt_pub) in its pgoutput configuration.

API

Member	Purpose
AddExternalSlot(name, configure)	Declare an external pgoutput slot named name.
WithPublication(name)	Override the publication name (default "{slot}_pub").
ForTable(table) / ForTable(schema, table)	Add a table by name (schema defaults to public).
ForEntity<T>()	Add the table mapped to T, resolved against the EF Core model.

At least one table is required (a pgoutput publication needs tables). Slot and publication names must be distinct from Wallaby's own slot/publication and from each other.

Lifecycle & semantics

• Leader-only and idempotent. External slots are created during the same self-config step as Wallaby's own slot - on the leader, before streaming, on every leadership acquisition. Creating a missing slot or re-applying a publication is safe to repeat.

• Reconciled. On each startup Wallaby reconciles the external publication's table set to your declared list (ALTER PUBLICATION ... ADD/DROP TABLE). Wallaby owns the table set: a table added to the publication out-of-band is dropped on the next run. Manage membership through AddExternalSlot.

• Pre-existing slots are adopted (and validated). If a slot with the declared name already exists, Wallaby reuses it rather than recreating it, and records it in wallaby.slot_registry. It fails fast if that slot isn't a pgoutput logical slot (e.g. a physical slot or one on a different output plugin), so a name clash with an unrelated slot surfaces as a clear startup error rather than a silent mismatch.

• Never auto-dropped. Removing an AddExternalSlot(...) declaration does not drop the slot or publication. A retired slot must be removedyourself:

  SELECT pg_drop_replication_slot('elt');
  DROP PUBLICATION elt_pub;

• Slot headroom. Wallaby's startup validation accounts for every slot it will create (its own plus all external ones) and fails fast with max_replication_slots guidance if there isn't room.

• Bookkeeping. Each provisioned slot is recorded in wallaby.slot_registry; external slots are marked kind = 'external' (Wallaby's own slot is 'primary').

Inactive until consumed

An external slot is created inactive and pins WAL the moment it exists. If nothing consumes it, WAL accumulates on the server. Only declare slots an external tool will actually read.

Scope

Wallaby only provisions external slots, it does not consume them, monitor their lag, or manage roles and grants for the external tool. Only pgoutput consumers are supported (the wal2json / test_decoding plugins are out of scope).