Artifact 849df9860df602dc2c55163d658c6b138213122f:
File www/selfcheck.html part of check-in [b807acf62e] - Documentation updates by drh on 2007-07-24 12:52:32.
0000: 3c 68 74 6d 6c 3e 0a 3c 68 65 61 64 3e 0a 3c 74 <html>.<head>.<t 0010: 69 74 6c 65 3e 46 6f 73 73 69 6c 20 52 65 70 6f itle>Fossil Repo 0020: 73 69 74 6f 72 79 20 49 6e 74 65 67 72 69 74 79 sitory Integrity 0030: 20 53 65 6c 66 2d 43 68 65 63 6b 73 3c 2f 74 69 Self-Checks</ti 0040: 74 6c 65 3e 0a 3c 2f 68 65 61 64 3e 0a 3c 62 6f tle>.</head>.<bo 0050: 64 79 20 62 67 63 6f 6c 6f 72 3d 22 77 68 69 74 dy bgcolor="whit 0060: 65 22 3e 0a 3c 68 31 20 61 6c 69 67 6e 3d 22 63 e">.<h1 align="c 0070: 65 6e 74 65 72 22 3e 0a 46 6f 73 73 69 6c 20 52 enter">.Fossil R 0080: 65 70 6f 73 69 74 6f 72 79 20 49 6e 74 65 67 72 epository Integr 0090: 69 74 79 20 53 65 6c 66 2d 43 68 65 63 6b 73 0a ity Self-Checks. 00a0: 3c 2f 68 31 3e 0a 0a 3c 70 3e 0a 45 76 65 6e 20 </h1>..<p>.Even 00b0: 74 68 6f 75 67 68 20 66 6f 73 73 69 6c 20 69 73 though fossil is 00c0: 20 61 20 72 65 6c 61 74 69 76 65 6c 79 20 6e 65 a relatively ne 00d0: 77 20 70 72 6f 6a 65 63 74 20 61 6e 64 20 73 74 w project and st 00e0: 69 6c 6c 20 63 6f 6e 74 61 69 6e 73 0a 6d 61 6e ill contains.man 00f0: 79 20 62 75 67 73 2c 20 69 74 20 69 73 20 64 65 y bugs, it is de 0100: 73 69 67 6e 65 64 20 77 69 74 68 20 66 65 61 74 signed with feat 0110: 75 72 65 73 20 74 6f 20 67 69 76 65 20 69 74 20 ures to give it 0120: 61 20 68 69 67 68 20 6c 65 76 65 6c 0a 6f 66 20 a high level.of 0130: 69 6e 74 65 67 72 69 74 79 20 73 6f 20 74 68 61 integrity so tha 0140: 74 20 79 6f 75 20 63 61 6e 20 68 61 76 65 20 63 t you can have c 0150: 6f 6e 66 69 64 65 6e 63 65 20 74 68 61 74 20 79 onfidence that y 0160: 6f 75 20 77 69 6c 6c 20 6e 6f 74 0a 6c 6f 73 65 ou will not.lose 0170: 20 79 6f 75 72 20 66 69 6c 65 73 2e 20 20 54 68 your files. Th 0180: 69 73 20 6e 6f 74 65 20 64 65 73 63 72 69 62 65 is note describe 0190: 73 20 74 68 65 20 64 65 66 65 6e 73 69 76 65 20 s the defensive 01a0: 6d 65 61 73 75 72 65 73 20 74 68 61 74 0a 66 6f measures that.fo 01b0: 73 73 69 6c 20 75 73 65 73 20 74 6f 20 68 65 6c ssil uses to hel 01c0: 70 20 70 72 65 76 65 6e 74 20 66 69 6c 65 20 6c p prevent file l 01d0: 6f 73 73 20 64 75 65 20 74 6f 20 62 75 67 73 2e oss due to bugs. 01e0: 0a 3c 2f 70 3e 0a 0a 3c 68 32 3e 41 74 6f 6d 69 .</p>..<h2>Atomi 01f0: 63 20 43 68 65 63 6b 2d 69 6e 73 20 57 69 74 68 c Check-ins With 0200: 20 52 6f 6c 6c 62 61 63 6b 3c 2f 68 32 3e 0a 0a Rollback</h2>.. 0210: 3c 70 3e 0a 54 68 65 20 66 6f 73 73 69 6c 20 72 <p>.The fossil r 0220: 65 70 6f 73 69 74 6f 72 79 20 69 73 20 61 6e 0a epository is an. 0230: 3c 61 20 68 72 65 66 3d 22 68 74 74 70 3a 2f 2f <a href="http:// 0240: 77 77 77 2e 73 71 6c 69 74 65 2e 6f 72 67 2f 22 www.sqlite.org/" 0250: 3e 53 51 4c 69 74 65 20 76 65 72 73 69 6f 6e 20 >SQLite version 0260: 33 3c 2f 61 3e 20 64 61 74 61 62 61 73 65 20 66 3</a> database f 0270: 69 6c 65 2e 20 20 0a 53 51 4c 69 74 65 20 69 73 ile. .SQLite is 0280: 20 76 65 72 79 20 6d 61 74 75 72 65 20 61 6e 64 very mature and 0290: 20 73 74 61 62 6c 65 20 61 6e 64 20 68 61 73 20 stable and has 02a0: 62 65 65 6e 20 69 6e 20 77 69 64 65 2d 73 70 72 been in wide-spr 02b0: 65 61 64 20 75 73 65 20 66 6f 72 20 6d 61 6e 79 ead use for many 02c0: 0a 79 65 61 72 73 2c 20 73 6f 20 77 65 20 68 61 .years, so we ha 02d0: 76 65 20 6c 69 74 74 6c 65 20 77 6f 72 72 69 65 ve little worrie 02e0: 73 20 74 68 61 74 20 69 74 20 6d 69 67 68 74 20 s that it might 02f0: 63 61 75 73 65 20 72 65 70 6f 73 69 74 6f 72 79 cause repository 0300: 0a 63 6f 72 72 75 70 74 69 6f 6e 2e 20 20 53 51 .corruption. SQ 0310: 4c 69 74 65 0a 64 61 74 61 62 61 73 65 73 20 64 Lite.databases d 0320: 6f 20 6e 6f 74 20 63 6f 72 72 75 70 74 20 65 76 o not corrupt ev 0330: 65 6e 20 69 66 20 61 20 70 72 6f 67 72 61 6d 20 en if a program 0340: 6f 72 20 73 79 73 74 65 6d 20 63 72 61 73 68 20 or system crash 0350: 6f 72 20 70 6f 77 65 72 0a 66 61 69 6c 75 72 65 or power.failure 0360: 20 6f 63 63 75 72 73 20 69 6e 20 74 68 65 20 6d occurs in the m 0370: 69 64 64 6c 65 20 6f 66 20 74 68 65 20 75 70 64 iddle of the upd 0380: 61 74 65 2e 20 20 49 66 20 73 6f 6d 65 20 6b 69 ate. If some ki 0390: 6e 64 20 6f 66 20 63 72 61 73 68 0a 64 6f 65 73 nd of crash.does 03a0: 20 6f 63 63 75 72 20 69 6e 20 74 68 65 20 6d 69 occur in the mi 03b0: 64 64 6c 65 20 6f 66 20 61 20 63 68 61 6e 67 65 ddle of a change 03c0: 2c 20 74 68 65 6e 20 61 6c 6c 20 74 68 65 20 63 , then all the c 03d0: 68 61 6e 67 65 73 20 61 72 65 20 72 6f 6c 6c 65 hanges are rolle 03e0: 64 0a 62 61 63 6b 20 74 68 65 20 6e 65 78 74 20 d.back the next 03f0: 74 69 6d 65 20 74 68 61 74 20 74 68 65 20 64 61 time that the da 0400: 74 61 62 61 73 65 20 69 73 20 61 63 63 65 73 73 tabase is access 0410: 65 64 2e 0a 3c 2f 70 3e 0a 0a 3c 70 3e 0a 41 20 ed..</p>..<p>.A 0420: 63 68 65 63 6b 2d 69 6e 20 6f 70 65 72 61 74 69 check-in operati 0430: 6f 6e 20 69 6e 20 66 6f 73 73 69 6c 20 6d 61 6b on in fossil mak 0440: 65 73 20 6d 61 6e 79 20 63 68 61 6e 67 65 73 20 es many changes 0450: 74 6f 20 74 68 65 20 72 65 70 6f 73 69 74 6f 72 to the repositor 0460: 79 0a 64 61 74 61 62 61 73 65 2e 20 20 42 75 74 y.database. But 0470: 20 61 6c 6c 20 74 68 65 73 65 20 63 68 61 6e 67 all these chang 0480: 65 73 20 68 61 70 70 65 6e 20 77 69 74 68 69 6e es happen within 0490: 20 61 20 73 69 6e 67 6c 65 20 74 72 61 6e 73 61 a single transa 04a0: 63 74 69 6f 6e 2e 0a 49 66 20 73 6f 6d 65 74 68 ction..If someth 04b0: 69 6e 67 20 67 6f 65 73 20 77 72 6f 6e 67 20 69 ing goes wrong i 04c0: 6e 20 74 68 65 20 6d 69 64 64 6c 65 20 6f 66 20 n the middle of 04d0: 74 68 65 20 63 6f 6d 6d 69 74 2c 20 74 68 65 6e the commit, then 04e0: 20 74 68 65 20 74 72 61 6e 73 61 63 74 69 6f 6e the transaction 04f0: 0a 69 73 20 72 6f 6c 6c 65 64 20 62 61 63 6b 20 .is rolled back 0500: 61 6e 64 20 74 68 65 20 64 61 74 61 62 61 73 65 and the database 0510: 20 69 73 20 75 6e 63 68 61 6e 67 65 64 2e 0a 3c is unchanged..< 0520: 2f 70 3e 0a 0a 3c 68 32 3e 56 65 72 69 66 69 63 /p>..<h2>Verific 0530: 61 74 69 6f 6e 20 4f 66 20 44 65 6c 74 61 20 45 ation Of Delta E 0540: 6e 63 6f 64 69 6e 67 73 20 50 72 69 6f 72 20 54 ncodings Prior T 0550: 6f 20 54 72 61 6e 73 61 63 74 69 6f 6e 20 43 6f o Transaction Co 0560: 6d 6d 69 74 3c 2f 68 32 3e 0a 0a 3c 70 3e 0a 54 mmit</h2>..<p>.T 0570: 68 65 20 63 6f 6e 74 65 6e 74 20 66 69 6c 65 73 he content files 0580: 20 74 68 61 74 20 63 6f 6d 70 72 69 73 65 20 74 that comprise t 0590: 68 65 20 67 6c 6f 62 61 6c 20 73 74 61 74 65 20 he global state 05a0: 6f 66 20 61 20 66 6f 73 73 69 6c 20 72 65 73 70 of a fossil resp 05b0: 6f 73 69 74 6f 72 79 0a 61 72 65 20 73 74 6f 72 ository.are stor 05c0: 65 64 20 69 6e 20 74 68 65 20 72 65 70 6f 73 69 ed in the reposi 05d0: 74 6f 72 79 20 61 73 20 61 20 74 72 65 65 2e 20 tory as a tree. 05e0: 20 54 68 65 20 6c 65 61 76 65 73 20 6f 66 20 74 The leaves of t 05f0: 68 65 20 74 72 65 65 20 61 72 65 0a 73 74 6f 72 he tree are.stor 0600: 65 64 20 61 73 20 7a 6c 69 62 2d 63 6f 6d 70 72 ed as zlib-compr 0610: 65 73 73 65 64 20 42 4c 4f 42 73 2e 20 20 49 6e essed BLOBs. In 0620: 74 65 72 69 6f 72 20 6e 6f 64 65 73 20 61 72 65 terior nodes are 0630: 20 64 65 6c 74 61 73 20 66 72 6f 6d 20 74 68 65 deltas from the 0640: 69 72 0a 64 65 63 65 6e 64 65 6e 74 73 2e 20 20 ir.decendents. 0650: 54 68 65 72 65 20 69 73 20 61 20 6c 6f 74 20 6f There is a lot o 0660: 66 20 65 6e 63 6f 64 69 6e 67 20 67 6f 69 6e 67 f encoding going 0670: 20 6f 6e 20 68 65 72 65 2e 20 20 54 68 65 72 65 on here. There 0680: 20 69 73 0a 7a 6c 69 62 2d 63 6f 6d 70 72 65 73 is.zlib-compres 0690: 73 69 6f 6e 20 77 68 69 63 68 20 69 73 20 72 65 sion which is re 06a0: 6c 61 74 69 76 65 6c 79 20 77 65 6c 6c 2d 74 65 latively well-te 06b0: 73 74 65 64 20 62 75 74 20 73 74 69 6c 6c 20 6d sted but still m 06c0: 69 67 68 74 0a 63 61 75 73 65 20 63 6f 72 72 75 ight.cause corru 06d0: 70 74 69 6f 6e 20 69 66 20 75 73 65 64 20 69 6d ption if used im 06e0: 70 72 6f 70 65 72 6c 79 2e 20 20 41 6e 64 20 74 properly. And t 06f0: 68 65 72 65 20 69 73 20 74 68 65 20 72 65 6c 61 here is the rela 0700: 74 69 76 65 6c 79 0a 6e 65 77 20 64 65 6c 74 61 tively.new delta 0710: 2d 65 6e 63 6f 64 69 6e 67 20 6d 65 63 68 61 6e -encoding mechan 0720: 69 73 6d 20 64 65 73 69 67 6e 65 64 20 65 78 70 ism designed exp 0730: 72 65 73 73 6c 79 20 66 6f 72 20 66 6f 73 73 69 ressly for fossi 0740: 6c 2e 20 20 57 65 20 77 61 6e 74 0a 74 6f 20 6d l. We want.to m 0750: 61 6b 65 20 73 75 72 65 20 74 68 61 74 20 62 75 ake sure that bu 0760: 67 73 20 69 6e 20 74 68 65 73 65 20 65 6e 63 6f gs in these enco 0770: 64 69 6e 67 20 6d 65 63 68 61 6e 69 73 6d 73 20 ding mechanisms 0780: 64 6f 20 6e 6f 74 20 6c 65 61 64 20 74 6f 0a 6c do not lead to.l 0790: 6f 73 73 20 6f 66 20 64 61 74 61 2e 0a 3c 2f 70 oss of data..</p 07a0: 3e 0a 0a 3c 70 3e 0a 54 6f 20 69 6e 63 72 65 61 >..<p>.To increa 07b0: 73 65 20 6f 75 72 20 63 6f 6e 66 69 64 65 6e 63 se our confidenc 07c0: 65 20 74 68 61 74 20 65 76 65 72 79 74 68 69 6e e that everythin 07d0: 67 20 69 6e 20 74 68 65 20 72 65 70 6f 73 69 74 g in the reposit 07e0: 6f 72 79 20 69 73 0a 72 65 63 6f 76 65 72 61 62 ory is.recoverab 07f0: 6c 65 2c 20 66 6f 73 73 69 6c 20 6d 61 6b 65 73 le, fossil makes 0800: 20 73 75 72 65 20 69 74 20 63 61 6e 20 65 78 74 sure it can ext 0810: 72 61 63 74 20 61 6e 20 65 78 61 63 74 20 72 65 ract an exact re 0820: 70 6c 69 63 61 74 65 0a 6f 66 20 65 76 65 72 79 plicate.of every 0830: 20 63 6f 6e 74 65 6e 74 20 66 69 6c 65 20 74 68 content file th 0840: 61 74 20 69 74 20 63 68 61 6e 67 65 73 20 6a 75 at it changes ju 0850: 73 74 20 70 72 69 6f 72 20 74 6f 20 74 72 61 6e st prior to tran 0860: 73 61 63 74 69 6f 6e 0a 63 6f 6d 6d 69 74 2e 20 saction.commit. 0870: 20 53 6f 20 64 75 72 69 6e 67 20 74 68 65 20 63 So during the c 0880: 6f 75 72 73 65 20 6f 66 20 63 68 65 63 6b 2d 69 ourse of check-i 0890: 6e 2c 20 6d 61 6e 79 20 64 69 66 66 65 72 65 6e n, many differen 08a0: 74 20 66 69 6c 65 73 0a 69 6e 20 74 68 65 20 72 t files.in the r 08b0: 65 70 6f 73 69 74 6f 72 79 20 6d 69 67 68 74 20 epository might 08c0: 62 65 20 6d 6f 64 69 66 69 65 64 2e 20 20 53 6f be modified. So 08d0: 6d 65 20 66 69 6c 65 73 20 61 72 65 20 73 69 6d me files are sim 08e0: 70 6c 79 0a 63 6f 6d 70 72 65 73 73 65 64 2e 20 ply.compressed. 08f0: 20 4f 74 68 65 72 20 66 69 6c 65 73 20 61 72 65 Other files are 0900: 20 64 65 6c 74 61 20 65 6e 63 6f 64 65 64 20 61 delta encoded a 0910: 6e 64 20 74 68 65 6e 20 63 6f 6d 70 72 65 73 73 nd then compress 0920: 65 64 2e 0a 57 68 69 6c 65 20 61 6c 6c 20 74 68 ed..While all th 0930: 69 73 20 69 73 20 67 6f 69 6e 67 20 6f 6e 2c 20 is is going on, 0940: 66 6f 73 73 69 6c 20 6d 61 6b 65 73 20 61 20 72 fossil makes a r 0950: 65 63 6f 72 64 20 6f 66 20 65 76 65 72 79 20 66 ecord of every f 0960: 69 6c 65 0a 74 68 61 74 20 69 73 20 65 6e 63 6f ile.that is enco 0970: 64 65 64 20 61 6e 64 20 74 68 65 20 53 48 41 31 ded and the SHA1 0980: 20 68 61 73 68 20 6f 66 20 74 68 65 20 6f 72 69 hash of the ori 0990: 67 69 6e 61 6c 20 63 6f 6e 74 65 6e 74 20 6f 66 ginal content of 09a0: 20 74 68 61 74 0a 66 69 6c 65 2e 20 20 54 68 65 that.file. 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