X-Git-Url: http://git.ozlabs.org/?p=ccan;a=blobdiff_plain;f=ccan%2Ftdb2%2Fdoc%2Fdesign.lyx%2Cv;h=70fe70e2fb431b548af941abbf08b51e9fe01d6c;hp=472202679897e06d877515fdff4cd58dc81fdb0e;hb=b8d05b195bfa10cb2a5b21985536ea45350029d5;hpb=95458bafc9dc99ac8fcd68aa8f48a9fc564e6a31

diff --git a/ccan/tdb2/doc/design.lyx,v b/ccan/tdb2/doc/design.lyx,v
index 47220267..70fe70e2 100644
--- a/ccan/tdb2/doc/design.lyx,v
+++ b/ccan/tdb2/doc/design.lyx,v
@@ -1,10 +1,20 @@
-head	1.8;
+head	1.10;
 access;
 symbols;
 locks; strict;
 comment	@# @;
 
 
+1.10
+date	2010.09.14.00.33.57;	author rusty;	state Exp;
+branches;
+next	1.9;
+
+1.9
+date	2010.09.09.07.25.12;	author rusty;	state Exp;
+branches;
+next	1.8;
+
 1.8
 date	2010.09.02.02.29.05;	author rusty;	state Exp;
 branches;
@@ -51,9 +61,9 @@ desc
 @
 
 
-1.8
+1.10
 log
-@Remove bogus footnote
+@Tracing attribute, talloc support.
 @
 text
 @#LyX 1.6.5 created this file. For more info see http://www.lyx.org/
@@ -111,8 +121,8 @@ Rusty Russell, IBM Corporation
 
 \change_deleted 0 1283307542
 26-July
-\change_inserted 0 1283307544
-1-September
+\change_inserted 0 1284423485
+14-September
 \change_unchanged
 -2010
 \end_layout
@@ -534,6 +544,17 @@ The tdb_open() call was expanded to tdb_open_ex(), which added an optional
 
 \begin_layout Subsubsection
 Proposed Solution
+\change_inserted 0 1284422789
+
+\begin_inset CommandInset label
+LatexCommand label
+name "attributes"
+
+\end_inset
+
+
+\change_unchanged
+
 \end_layout
 
 \begin_layout Standard
@@ -893,6 +914,18 @@ Internal locking is required to make sure that fcntl locks do not overlap
 \begin_layout Standard
 The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe
  version of the library, and otherwise no overhead will exist.
+
+\change_inserted 0 1284016998
+ Alternatively, a hooking mechanism similar to that proposed for 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "Proposed-Solution-locking-hook"
+
+\end_inset
+
+ could be used to enable pthread locking at runtime.
+\change_unchanged
+
 \end_layout
 
 \begin_layout Subsection
@@ -1241,6 +1274,165 @@ reference "TDB_CLEAR_IF_FIRST-Imposes-Performance"
 \end_inset
 
 .
+\change_inserted 0 1284015637
+
+\end_layout
+
+\begin_layout Subsection
+
+\change_inserted 0 1284015716
+Extending The Header Is Difficult
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284015906
+We have reserved (zeroed) words in the TDB header, which can be used for
+ future features.
+ If the future features are compulsory, the version number must be updated
+ to prevent old code from accessing the database.
+ But if the future feature is optional, we have no way of telling if older
+ code is accessing the database or not.
+\end_layout
+
+\begin_layout Subsubsection
+
+\change_inserted 0 1284015637
+Proposed Solution
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284016114
+The header should contain a 
+\begin_inset Quotes eld
+\end_inset
+
+format variant
+\begin_inset Quotes erd
+\end_inset
+
+ value (64-bit).
+ This is divided into two 32-bit parts:
+\end_layout
+
+\begin_layout Enumerate
+
+\change_inserted 0 1284016149
+The lower part reflects the format variant understood by code accessing
+ the database.
+\end_layout
+
+\begin_layout Enumerate
+
+\change_inserted 0 1284016639
+The upper part reflects the format variant you must understand to write
+ to the database (otherwise you can only open for reading).
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284016821
+The latter field can only be written at creation time, the former should
+ be written under the OPEN_LOCK when opening the database for writing, if
+ the variant of the code is lower than the current lowest variant.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284016803
+This should allow backwards-compatible features to be added, and detection
+ if older code (which doesn't understand the feature) writes to the database.
+\change_deleted 0 1284016101
+
+\end_layout
+
+\begin_layout Subsection
+
+\change_inserted 0 1284015634
+Record Headers Are Not Expandible
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284015634
+If we later want to add (say) checksums on keys and data, it would require
+ another format change, which we'd like to avoid.
+\end_layout
+
+\begin_layout Subsubsection
+
+\change_inserted 0 1284015634
+Proposed Solution
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284422552
+We often have extra padding at the tail of a record.
+ If we ensure that the first byte (if any) of this padding is zero, we will
+ have a way for future changes to detect code which doesn't understand a
+ new format: the new code would write (say) a 1 at the tail, and thus if
+ there is no tail or the first byte is 0, we would know the extension is
+ not present on that record.
+\end_layout
+
+\begin_layout Subsection
+
+\change_inserted 0 1284422568
+TDB Does Not Use Talloc
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284422646
+Many users of TDB (particularly Samba) use the talloc allocator, and thus
+ have to wrap TDB in a talloc context to use it conveniently.
+\end_layout
+
+\begin_layout Subsubsection
+
+\change_inserted 0 1284422656
+Proposed Solution
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284423065
+The allocation within TDB is not complicated enough to justify the use of
+ talloc, and I am reluctant to force another (excellent) library on TDB
+ users.
+ Nonetheless a compromise is possible.
+ An attribute (see 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "attributes"
+
+\end_inset
+
+) can be added later to tdb_open() to provide an alternate allocation mechanism,
+ specifically for talloc but usable by any other allocator (which would
+ ignore the 
+\begin_inset Quotes eld
+\end_inset
+
+context
+\begin_inset Quotes erd
+\end_inset
+
+ argument).
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284423042
+This would form a talloc heirarchy as expected, but the caller would still
+ have to attach a destructor to the tdb context returned from tdb_open to
+ close it.
+ All TDB_DATA fields would be children of the tdb_context, and the caller
+ would still have to manage them (using talloc_free() or talloc_steal()).
+\change_unchanged
+
 \end_layout
 
 \begin_layout Section
@@ -1818,7 +2010,7 @@ status open
 
 \begin_layout Plain Layout
 
-\change_inserted 0 1283310945
+\change_inserted 0 1284424151
 Using 
 \begin_inset Formula $2^{16+N*3}$
 \end_inset
@@ -1829,6 +2021,8 @@ means 0 gives a minimal 65536-byte zone, 15 gives the maximal
 
  byte zone.
  Zones range in factor of 8 steps.
+ Given the zone size for the zone the current record is in, we can determine
+ the start of the zone.
 \change_unchanged
 
 \end_layout
@@ -2273,6 +2467,8 @@ TDB Does Not Have Snapshot Support
 
 \begin_layout Subsubsection
 Proposed Solution
+\change_deleted 0 1284423472
+
 \end_layout
 
 \begin_layout Standard
@@ -2285,7 +2481,23 @@ use a real database
 \begin_inset Quotes erd
 \end_inset
 
+
+\change_inserted 0 1284423891
+ 
+\change_deleted 0 1284423891
 .
+
+\change_inserted 0 1284423901
+ (but see 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "replay-attribute"
+
+\end_inset
+
+).
+\change_unchanged
+
 \end_layout
 
 \begin_layout Standard
@@ -2308,6 +2520,8 @@ This would not allow arbitrary changes to the database, such as tdb_repack
 \begin_layout Standard
 We could then implement snapshots using a similar method, using multiple
  different hash tables/free tables.
+\change_inserted 0 1284423495
+
 \end_layout
 
 \begin_layout Subsection
@@ -2327,6 +2541,18 @@ Proposed Solution
 \end_layout
 
 \begin_layout Standard
+
+\change_inserted 0 1284424201
+None (but see 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "replay-attribute"
+
+\end_inset
+
+).
+ 
+\change_unchanged
 We could solve a small part of the problem by providing read-only transactions.
  These would allow one write transaction to begin, but it could not commit
  until all r/o transactions are done.
@@ -2512,6 +2738,53 @@ At some later point, a sync would allow recovery of the old data into the
  free lists (perhaps when the array of top-level pointers filled).
  On crash, tdb_open() would examine the array of top levels, and apply the
  transactions until it encountered an invalid checksum.
+\change_inserted 0 1284423555
+
+\end_layout
+
+\begin_layout Subsection
+
+\change_inserted 0 1284423617
+Tracing Is Fragile, Replay Is External
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284423719
+The current TDB has compile-time-enabled tracing code, but it often breaks
+ as it is not enabled by default.
+ In a similar way, the ctdb code has an external wrapper which does replay
+ tracing so it can coordinate cluster-wide transactions.
+\end_layout
+
+\begin_layout Subsubsection
+
+\change_inserted 0 1284423864
+Proposed Solution
+\begin_inset CommandInset label
+LatexCommand label
+name "replay-attribute"
+
+\end_inset
+
+
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1284423850
+Tridge points out that an attribute can be later added to tdb_open (see
+ 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "attributes"
+
+\end_inset
+
+) to provide replay/trace hooks, which could become the basis for this and
+ future parallel transactions and snapshot support.
+\change_unchanged
+
 \end_layout
 
 \end_body
@@ -2519,6 +2792,47 @@ At some later point, a sync would allow recovery of the old data into the
 @
 
 
+1.9
+log
+@Extension mechanism.
+@
+text
+@d56 2
+a57 2
+\change_inserted 0 1284016854
+9-September
+d479 11
+d1303 1
+a1303 1
+\change_inserted 0 1284016847
+d1310 56
+d1945 1
+a1945 1
+\change_inserted 0 1283310945
+d1956 2
+d2402 2
+d2416 4
+d2421 12
+d2455 2
+d2476 12
+d2673 47
+@
+
+
+1.8
+log
+@Remove bogus footnote
+@
+text
+@d56 2
+a57 2
+\change_inserted 0 1283307544
+1-September
+d838 12
+d1198 103
+@
+
+
 1.7
 log
 @Moving hash table does not work.