Which Replicated Tables Got Changes in BDR Replication

My favourite feature of BDR by 2ndQuadrant is “multi master table level selective replication”. Currently, I am preparing a plan to switch from trigger based to low level BDR replication for better performance.

But, how can I detect which replicated tables got changes (needed for memcached invalidation, for example). I can not trap it inside always-enabled-trigger any longer. An alternative is to modify BDR plugin c source code. Thanks to 2ndQuadrant team for clear source code. Following code is my modification for bdr_apply.c:

1. Module level variable and function declaration:
#define DTABLECHUNK 5
static char** dtables;
static int dtablessize;
static int dtablescount;
static void append_dtables(char* t);

2. Variable initialization at the end of process_remote_begin:
static void
process_remote_begin(StringInfo s) {

...
dtables=NULL;
dtablessize=dtablescount=0;

}


3. Modify a bit declaration and definition of read_rel function to catch tablename:
static BDRRelation *
read_rel(StringInfo s, LOCKMODE mode, char** dtablename){

int relnamelen;
int nspnamelen;
RangeVar* rv;
Oid relid;
MemoryContext oldcontext;

rv = makeNode(RangeVar);

nspnamelen = pq_getmsgint(s, 2);
rv->schemaname = (char *) pq_getmsgbytes(s, nspnamelen);

relnamelen = pq_getmsgint(s, 2);
rv->relname = (char *) pq_getmsgbytes(s, relnamelen);

oldcontext=MemoryContextSwitchTo(MessageContext);
//nspnamelen and relnamelen is the length of the name including terminating zero
*dtablename=(char*)palloc( (nspnamelen+relnamelen) * sizeof(char) );
MemoryContextSwitchTo(oldcontext);
sprintf(*dtablename, "%s.%s", rv->schemaname, rv->relname);

relid = RangeVarGetRelidExtended(rv, mode, false, false, NULL, NULL);

return bdr_heap_open(relid, NoLock);

}

4. Declare variable char *tablename inside process_remote_insert, process_remote_update and process_remote_delete. Add its address (&tablename) as third argument for read_real function invocation. Then, register the tablename by calling my append_dtables just after simple_heap_insert, simple_heap_update and simple_heap_delete.
5. Call outside function to notify which tables got changes inside process_remote_commit function, somewhere after a call to CommitTransactionCommand.
for(i=0;dtables!=NULL && i<dtablescount;i++) {
//any function call to notify dtables[i] got changes
}

6. And finally, this is my append_dtables definition. Note that memory is allocated in MessageContext so it will be freed automatically in bdr_apply_work main loop.
static void
append_dtables(char* t) {

int i, allocsize;
MemoryContext oldcontext;

for(i=0;i<dtablescount;i++) {

if(!strcmp(dtables[i], t)) {//ensure unique table registration

oldcontext=MemoryContextSwitchTo(MessageContext);
pfree(t);
MemoryContextSwitchTo(oldcontext);
return;

}

}
if(dtablessize<++dtablescount) {

dtablessize+=DTABLECHUNK;
allocsize=dtablessize * sizeof(char*);
oldcontext=MemoryContextSwitchTo(MessageContext);
if(dtables==NULL)

dtables=(char*)palloc(allocsize);

else

dtables=(char**)repalloc(dtables, allocsize);

MemoryContextSwitchTo(oldcontext);

}
dtables[dtablescount-1]=t;

}


What I love from open source project is that there is always a way to tweak to suit my development framework.

• BDR table changes