Commment updates.

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index bb51e4ee91ec8eecf60cd62ab797c079b93ebb43..87e4abf1f27500465af122226c34ea9e226ea4f5 100644 (file)
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -641,8 +641,7 @@ StartReplication(StartReplicationCmd *cmd)
 }
 
 /*
- * Initialize a logical or physical replication slot and wait for an initial
- * consistent point to start sending changes from.
+ * Create a new replication slot.
  */
 static void
 CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
@@ -662,11 +661,19 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 
        slot_name = NameStr(MyReplicationSlot->data.name);
 
+       /*
+        * It may seem somewhat pointless to send back the same slot name the
+        * client just requested and nothing else, but logical replication
+        * will add more fields here.  (We could consider removing the slot
+        * name from what's sent back, though, since the client has specified
+        * that.)
+        */
+
        pq_beginmessage(&buf, 'T');
        pq_sendint(&buf, 1, 2);         /* 1 field */
 
        /* first field: slot name */
-       pq_sendstring(&buf, "replication_id");  /* col name */
+       pq_sendstring(&buf, "slot_name");       /* col name */
        pq_sendint(&buf, 0, 4);         /* table oid */
        pq_sendint(&buf, 0, 2);         /* attnum */
        pq_sendint(&buf, TEXTOID, 4);           /* type oid */
@@ -693,15 +700,15 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 }
 
 /*
- * Free permanent state by a now inactive but defined logical slot.
+ * Get rid of a replication slot that is no longer wanted.
  */
 static void
 DropReplicationSlot(DropReplicationSlotCmd *cmd)
 {
-       /* no need to check decoding requirements here */;
        ReplicationSlotDrop(cmd->slotname);
        EndCommand("DROP_REPLICATION_SLOT", DestRemote);
 }
+
 /*
  * Execute an incoming replication command.
  */

diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index dffbc5462fa8e383112262b0bfc164f826f08f0a..2c85b54c31237512c743ac46ca4c895da5625d65 100644 (file)
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -23,22 +23,15 @@ typedef struct ReplicationSlotPersistentData
        /* database the slot is active on */
        Oid                     database;
 
-       /* xmin horizon for data */
-       TransactionId data_xmin;
-
-       /* ----
-        * For logical decoding, this contains the point where, after a shutdown
-        * or crash, the to have to restart decoding from to
-        * a) find a valid & ready snapshot
-        * b) the complete content for all in-progress xacts
-        *
-        * For streaming replication, this contains the oldest LSN (in any
-        * timeline) the standb might ask for.
+       /*
+        * xmin horizon for data
         *
-        * For both only WAL segments that are smaller than restart_decoding, will
-        * be removed.
-        * ----
+        * NB: This may represent a value that hasn't been written to disk yet;
+        * see notes for effective_data_xmin, below.
         */
+       TransactionId data_xmin;
+
+       /* oldest LSN that might be required by this replication slot */
        XLogRecPtr      restart_lsn;
 
 } ReplicationSlotPersistentData;
@@ -57,12 +50,21 @@ typedef struct ReplicationSlot
        /* is somebody streaming out changes for this slot */
        bool            active;
 
-       /* data surviving shutdowns and crashes */
-       ReplicationSlotPersistentData data;
-
-       /* in-memory xmin horizon, updated after syncing to disk, used for computations */
+       /*
+        * For logical decoding, it's extremely important that we never remove any
+        * data that's still needed for decoding purposes, even after a crash;
+        * otherwise, decoding will produce wrong answers.  Ordinary streaming
+        * replication also needs to prevent old row versions from being removed
+        * too soon, but the worst consequence we might encounter there is unwanted
+        * query cancellations on the standby.  Thus, for logical decoding,
+        * this value represents the latest data_xmin that has actually been
+        * written to disk, whereas for streaming replication, it's just the
+        * same as the persistent value (data.data_xmin).
+        */
        TransactionId effective_data_xmin;
 
+       /* data surviving shutdowns and crashes */
+       ReplicationSlotPersistentData data;
 } ReplicationSlot;
 
 /*