X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=lib%2Fovsdb-idl.h;h=27008b762384851b043492e32339f450c2e71f86;hb=d86a6c099ff4a1e0c7d1b437fa4ff0a9684fcdb9;hp=34ccf061931cb7542f56a2e7e7f72f551347d2aa;hpb=94fbe1aae29ecb71241cde7a8ed2688fa4621e1d;p=openvswitch

diff --git a/lib/ovsdb-idl.h b/lib/ovsdb-idl.h
index 34ccf061..27008b76 100644
--- a/lib/ovsdb-idl.h
+++ b/lib/ovsdb-idl.h
@@ -1,4 +1,4 @@
-/* Copyright (c) 2009, 2010, 2011, 2012 Nicira Networks.
+/* Copyright (c) 2009, 2010, 2011, 2012 Nicira, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -57,6 +57,7 @@ bool ovsdb_idl_is_lock_contended(const struct ovsdb_idl *);
 unsigned int ovsdb_idl_get_seqno(const struct ovsdb_idl *);
 bool ovsdb_idl_has_ever_connected(const struct ovsdb_idl *);
 void ovsdb_idl_force_reconnect(struct ovsdb_idl *);
+void ovsdb_idl_verify_write_only(struct ovsdb_idl *);
 
 /* Choosing columns and tables to replicate. */
 
@@ -112,7 +113,56 @@ const struct ovsdb_datum *ovsdb_idl_get(const struct ovsdb_idl_row *,
 
 bool ovsdb_idl_row_is_synthetic(const struct ovsdb_idl_row *);
 
-/* Transactions. */
+/* Transactions.
+ *
+ * A transaction may modify the contents of a database by modifying the values
+ * of columns, deleting rows, inserting rows, or adding checks that columns in
+ * the database have not changed ("verify" operations), through
+ * ovsdb_idl_txn_*() functions.  (The OVSDB IDL code generator produces helper
+ * functions that internally call the ovsdb_idl_txn_*() functions.  These are
+ * likely to be more convenient.)
+ *
+ * Reading and writing columns and inserting and deleting rows are all
+ * straightforward.  The reasons to verify columns are less obvious.
+ * Verification is the key to maintaining transactional integrity.  Because
+ * OVSDB handles multiple clients, it can happen that between the time that
+ * OVSDB client A reads a column and writes a new value, OVSDB client B has
+ * written that column.  Client A's write should not ordinarily overwrite
+ * client B's, especially if the column in question is a "map" column that
+ * contains several more or less independent data items.  If client A adds a
+ * "verify" operation before it writes the column, then the transaction fails
+ * in case client B modifies it first.  Client A will then see the new value of
+ * the column and compose a new transaction based on the new contents written
+ * by client B.
+ *
+ * When a transaction is complete, which must be before the next call to
+ * ovsdb_idl_run() on 'idl', call ovsdb_idl_txn_commit() or
+ * ovsdb_idl_txn_abort().
+ *
+ * The life-cycle of a transaction looks like this:
+ *
+ * 1. Create the transaction and record the initial sequence number:
+ *
+ *     seqno = ovsdb_idl_get_seqno(idl);
+ *     txn = ovsdb_idl_txn_create(idl);
+ *
+ * 2. Modify the database with ovsdb_idl_txn_*() functions directly or
+ *    indirectly.
+ *
+ * 3. Commit the transaction by calling ovsdb_idl_txn_commit().  The first call
+ *    to this function probably returns TXN_INCOMPLETE.  The client must keep
+ *    calling again along as this remains true, calling ovsdb_idl_run() in
+ *    between to let the IDL do protocol processing.  (If the client doesn't
+ *    have anything else to do in the meantime, it can use
+ *    ovsdb_idl_txn_commit_block() to avoid having to loop itself.)
+ *
+ * 4. If the final status is TXN_TRY_AGAIN, wait for ovsdb_idl_get_seqno() to
+ *    change from the saved 'seqno' (it's possible that it's already changed,
+ *    in which case the client should not wait at all), then start over from
+ *    step 1.  Only a call to ovsdb_idl_run() will change the return value of
+ *    ovsdb_idl_get_seqno().  (ovsdb_idl_txn_commit_block() calls
+ *    ovsdb_idl_run().)
+ */
 
 enum ovsdb_idl_txn_status {
     TXN_UNCOMMITTED,            /* Not yet committed or aborted. */