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It's how you keep all errors and slow requests while throwing away the bulk of healthy traffic, the opposite of head sampling, which decides up front before knowing what happens.",[577,589,590,591,595,596,599,600,603,604,607],{},"The full theory and config reference, covering built-in ",[592,593,594],"code",{},"keep"," rules, custom predicates via ",[592,597,598],{},"evlog:emit:keep",", combining head + tail sampling, lives at ",[601,602,61],"a",{"href":62},". This page covers the ",[580,605,606],{},"extension surface",": how to plug your own keep logic into the pipeline.",[609,610,613,616,660],"prompt",{":actions":611,"description":612,"icon":64},"[\"copy\",\"cursor\",\"claude\"]","Configure tail sampling on evlog",[577,614,615],{},"Set up tail sampling so I keep all errors and slow requests while dropping healthy noise.",[617,618,619,626,640,643,653],"ul",{},[620,621,622,623],"li",{},"Start with the built-in declarative rules: ",[592,624,625],{},"evlog.sampling.keep = { status: '>=400', duration: '>1000', path: ['\u002Fapi\u002Fauth\u002F*'] }",[620,627,628,629,631,632,635,636,639],{},"For multi-field or derived conditions, register an ",[592,630,598],{}," hook (Nitro: ",[592,633,634],{},"nitroApp.hooks.hook('evlog:emit:keep', (ctx) => ...)","); set ",[592,637,638],{},"ctx.shouldKeep = true"," to keep",[620,641,642],{},"Keep the hook fast. It runs on every request after enrichment, so no I\u002FO and no async work",[620,644,645,646,649,650,652],{},"Combine with head sampling (e.g. 10% of healthy traffic) by setting both ",[592,647,648],{},"sample"," (head) and ",[592,651,594],{}," (tail)",[620,654,655,656,659],{},"Always keep error events (",[592,657,658],{},"level: 'error'",") regardless of sampling; double-check rules don't accidentally drop them",[577,661,662,663],{},"Docs: ",[601,664,665],{"href":665,"rel":666},"https:\u002F\u002Fwww.evlog.dev\u002Fextend\u002Ftail-sampling",[667],"nofollow",[669,670,672],"h2",{"id":671},"when-the-built-in-rules-arent-enough","When the built-in rules aren't enough",[577,674,675,676,678],{},"The built-in declarative ",[592,677,594],{}," rules cover the typical cases (status code thresholds, duration thresholds, path matching, level matching). Drop to a custom hook when you need:",[617,680,681,695,703],{},[620,682,683,686,687,690,691,694],{},[580,684,685],{},"Conditional logic on more than one field"," (e.g. \"keep if ",[592,688,689],{},"status >= 500"," AND ",[592,692,693],{},"user.plan === 'enterprise'","\")",[620,696,697,686,700,694],{},[580,698,699],{},"Keep based on a derived value",[592,701,702],{},"event.audit?.context.actor.role === 'admin'",[620,704,705,708],{},[580,706,707],{},"Stateful decisions"," (rare; needs care since sampling runs in the hot path)",[669,710,712],{"id":711},"custom-keep-hook","Custom keep hook",[577,714,715],{},"The hook signature is the same regardless of framework. The wiring depends on your runtime.",[717,718,719,878],"code-group",{},[720,721,727],"pre",{"className":722,"code":723,"filename":724,"language":725,"meta":726,"style":726},"language-ts shiki shiki-themes material-theme-lighter material-theme material-theme-palenight","nitroApp.hooks.hook('evlog:emit:keep', (ctx) => {\n  if (ctx.context.user?.plan === 'enterprise' && ctx.status >= 500) {\n    ctx.shouldKeep = true\n  }\n})\n","Nuxt \u002F Nitro","ts","",[592,728,729,782,845,863,869],{"__ignoreMap":726},[730,731,734,738,742,745,747,751,754,757,760,762,765,768,772,775,779],"span",{"class":732,"line":733},"line",1,[730,735,737],{"class":736},"sTEyZ","nitroApp",[730,739,741],{"class":740},"sMK4o",".",[730,743,744],{"class":736},"hooks",[730,746,741],{"class":740},[730,748,750],{"class":749},"s2Zo4","hook",[730,752,753],{"class":736},"(",[730,755,756],{"class":740},"'",[730,758,598],{"class":759},"sfazB",[730,761,756],{"class":740},[730,763,764],{"class":740},",",[730,766,767],{"class":740}," (",[730,769,771],{"class":770},"sHdIc","ctx",[730,773,774],{"class":740},")",[730,776,778],{"class":777},"spNyl"," =>",[730,780,781],{"class":740}," {\n",[730,783,785,789,792,794,796,799,801,804,807,810,813,816,819,821,824,827,829,832,835,839,842],{"class":732,"line":784},2,[730,786,788],{"class":787},"s7zQu","  if",[730,790,767],{"class":791},"swJcz",[730,793,771],{"class":736},[730,795,741],{"class":740},[730,797,798],{"class":736},"context",[730,800,741],{"class":740},[730,802,803],{"class":736},"user",[730,805,806],{"class":740},"?.",[730,808,809],{"class":736},"plan",[730,811,812],{"class":740}," ===",[730,814,815],{"class":740}," '",[730,817,818],{"class":759},"enterprise",[730,820,756],{"class":740},[730,822,823],{"class":740}," &&",[730,825,826],{"class":736}," ctx",[730,828,741],{"class":740},[730,830,831],{"class":736},"status",[730,833,834],{"class":740}," >=",[730,836,838],{"class":837},"sbssI"," 500",[730,840,841],{"class":791},") ",[730,843,844],{"class":740},"{\n",[730,846,848,851,853,856,859],{"class":732,"line":847},3,[730,849,850],{"class":736},"    ctx",[730,852,741],{"class":740},[730,854,855],{"class":736},"shouldKeep",[730,857,858],{"class":740}," =",[730,860,862],{"class":861},"sfNiH"," true\n",[730,864,866],{"class":732,"line":865},4,[730,867,868],{"class":740},"  }\n",[730,870,872,875],{"class":732,"line":871},5,[730,873,874],{"class":740},"}",[730,876,877],{"class":736},")\n",[720,879,882],{"className":722,"code":880,"filename":881,"language":725,"meta":726,"style":726},"import { definePlugin } from 'evlog\u002Ftoolkit'\n\nexport const keepEnterpriseErrors = definePlugin({\n  name: 'keep-enterprise-errors',\n  keep(ctx) {\n    if (ctx.context.user?.plan === 'enterprise' && ctx.status >= 500) {\n      ctx.shouldKeep = true\n    }\n  },\n})\n\n\u002F\u002F Then: initLogger({ plugins: [keepEnterpriseErrors] })\n\u002F\u002F   or: app.use(evlog({ plugins: [keepEnterpriseErrors] }))\n","Plugin (any framework)",[592,883,884,909,915,935,953,966,1012,1026,1032,1038,1045,1050,1057],{"__ignoreMap":726},[730,885,886,889,892,895,898,901,903,906],{"class":732,"line":733},[730,887,888],{"class":787},"import",[730,890,891],{"class":740}," {",[730,893,894],{"class":736}," definePlugin",[730,896,897],{"class":740}," }",[730,899,900],{"class":787}," from",[730,902,815],{"class":740},[730,904,905],{"class":759},"evlog\u002Ftoolkit",[730,907,908],{"class":740},"'\n",[730,910,911],{"class":732,"line":784},[730,912,914],{"emptyLinePlaceholder":913},true,"\n",[730,916,917,920,923,926,929,931,933],{"class":732,"line":847},[730,918,919],{"class":787},"export",[730,921,922],{"class":777}," const",[730,924,925],{"class":736}," keepEnterpriseErrors ",[730,927,928],{"class":740},"=",[730,930,894],{"class":749},[730,932,753],{"class":736},[730,934,844],{"class":740},[730,936,937,940,943,945,948,950],{"class":732,"line":865},[730,938,939],{"class":791},"  name",[730,941,942],{"class":740},":",[730,944,815],{"class":740},[730,946,947],{"class":759},"keep-enterprise-errors",[730,949,756],{"class":740},[730,951,952],{"class":740},",\n",[730,954,955,958,960,962,964],{"class":732,"line":871},[730,956,957],{"class":791},"  keep",[730,959,753],{"class":740},[730,961,771],{"class":770},[730,963,774],{"class":740},[730,965,781],{"class":740},[730,967,969,972,974,976,978,980,982,984,986,988,990,992,994,996,998,1000,1002,1004,1006,1008,1010],{"class":732,"line":968},6,[730,970,971],{"class":787},"    if",[730,973,767],{"class":791},[730,975,771],{"class":736},[730,977,741],{"class":740},[730,979,798],{"class":736},[730,981,741],{"class":740},[730,983,803],{"class":736},[730,985,806],{"class":740},[730,987,809],{"class":736},[730,989,812],{"class":740},[730,991,815],{"class":740},[730,993,818],{"class":759},[730,995,756],{"class":740},[730,997,823],{"class":740},[730,999,826],{"class":736},[730,1001,741],{"class":740},[730,1003,831],{"class":736},[730,1005,834],{"class":740},[730,1007,838],{"class":837},[730,1009,841],{"class":791},[730,1011,844],{"class":740},[730,1013,1015,1018,1020,1022,1024],{"class":732,"line":1014},7,[730,1016,1017],{"class":736},"      ctx",[730,1019,741],{"class":740},[730,1021,855],{"class":736},[730,1023,858],{"class":740},[730,1025,862],{"class":861},[730,1027,1029],{"class":732,"line":1028},8,[730,1030,1031],{"class":740},"    }\n",[730,1033,1035],{"class":732,"line":1034},9,[730,1036,1037],{"class":740},"  },\n",[730,1039,1041,1043],{"class":732,"line":1040},10,[730,1042,874],{"class":740},[730,1044,877],{"class":736},[730,1046,1048],{"class":732,"line":1047},11,[730,1049,914],{"emptyLinePlaceholder":913},[730,1051,1053],{"class":732,"line":1052},12,[730,1054,1056],{"class":1055},"sHwdD","\u002F\u002F Then: initLogger({ plugins: [keepEnterpriseErrors] })\n",[730,1058,1060],{"class":732,"line":1059},13,[730,1061,1062],{"class":1055},"\u002F\u002F   or: app.use(evlog({ plugins: [keepEnterpriseErrors] }))\n",[577,1064,1065],{},"For non-trivial logic, prefer the plugin shape, since it travels with the rest of your evlog config (drains, enrichers) and is reusable across frameworks.",[669,1067,1069],{"id":1068},"composing-several-keep-predicates","Composing several keep predicates",[577,1071,1072,1073,1076,1077,1079,1080,1082,1083,1086],{},"Use ",[592,1074,1075],{},"composeKeep"," from ",[592,1078,905],{}," to combine multiple predicates into one hook. Each predicate runs independently and the final ",[592,1081,855],{}," is ",[592,1084,1085],{},"true"," if any of them set it:",[720,1088,1090],{"className":722,"code":1089,"language":725,"meta":726,"style":726},"import { composeKeep } from 'evlog\u002Ftoolkit'\n\nconst keep = composeKeep([\n  ({ duration, shouldKeep }) => duration && duration > 2000 ? true : shouldKeep,\n  ({ event }) => event.level === 'error',\n  ({ context, status }) => context.user?.plan === 'enterprise' && status >= 500,\n])\n",[592,1091,1092,1111,1115,1130,1175,1205,1252],{"__ignoreMap":726},[730,1093,1094,1096,1098,1101,1103,1105,1107,1109],{"class":732,"line":733},[730,1095,888],{"class":787},[730,1097,891],{"class":740},[730,1099,1100],{"class":736}," composeKeep",[730,1102,897],{"class":740},[730,1104,900],{"class":787},[730,1106,815],{"class":740},[730,1108,905],{"class":759},[730,1110,908],{"class":740},[730,1112,1113],{"class":732,"line":784},[730,1114,914],{"emptyLinePlaceholder":913},[730,1116,1117,1120,1123,1125,1127],{"class":732,"line":847},[730,1118,1119],{"class":777},"const",[730,1121,1122],{"class":736}," keep ",[730,1124,928],{"class":740},[730,1126,1100],{"class":749},[730,1128,1129],{"class":736},"([\n",[730,1131,1132,1135,1138,1140,1143,1146,1148,1151,1154,1156,1159,1162,1165,1168,1171,1173],{"class":732,"line":865},[730,1133,1134],{"class":740},"  ({",[730,1136,1137],{"class":770}," duration",[730,1139,764],{"class":740},[730,1141,1142],{"class":770}," shouldKeep",[730,1144,1145],{"class":740}," })",[730,1147,778],{"class":777},[730,1149,1150],{"class":736}," duration ",[730,1152,1153],{"class":740},"&&",[730,1155,1150],{"class":736},[730,1157,1158],{"class":740},">",[730,1160,1161],{"class":837}," 2000",[730,1163,1164],{"class":740}," ?",[730,1166,1167],{"class":861}," true",[730,1169,1170],{"class":740}," :",[730,1172,1142],{"class":736},[730,1174,952],{"class":740},[730,1176,1177,1179,1182,1184,1186,1188,1190,1193,1196,1198,1201,1203],{"class":732,"line":871},[730,1178,1134],{"class":740},[730,1180,1181],{"class":770}," event",[730,1183,1145],{"class":740},[730,1185,778],{"class":777},[730,1187,1181],{"class":736},[730,1189,741],{"class":740},[730,1191,1192],{"class":736},"level ",[730,1194,1195],{"class":740},"===",[730,1197,815],{"class":740},[730,1199,1200],{"class":759},"error",[730,1202,756],{"class":740},[730,1204,952],{"class":740},[730,1206,1207,1209,1212,1214,1217,1219,1221,1223,1225,1227,1229,1232,1234,1236,1238,1240,1242,1245,1248,1250],{"class":732,"line":968},[730,1208,1134],{"class":740},[730,1210,1211],{"class":770}," context",[730,1213,764],{"class":740},[730,1215,1216],{"class":770}," status",[730,1218,1145],{"class":740},[730,1220,778],{"class":777},[730,1222,1211],{"class":736},[730,1224,741],{"class":740},[730,1226,803],{"class":736},[730,1228,806],{"class":740},[730,1230,1231],{"class":736},"plan ",[730,1233,1195],{"class":740},[730,1235,815],{"class":740},[730,1237,818],{"class":759},[730,1239,756],{"class":740},[730,1241,823],{"class":740},[730,1243,1244],{"class":736}," status ",[730,1246,1247],{"class":740},">=",[730,1249,838],{"class":837},[730,1251,952],{"class":740},[730,1253,1254],{"class":732,"line":1014},[730,1255,1256],{"class":736},"])\n",[577,1258,1259,1260,1263],{},"Errors in individual predicates are isolated (logged with the ",[592,1261,1262],{},"[evlog\u002Fkeep]"," prefix) so a buggy predicate cannot silently drop legitimate events.",[669,1265,1267],{"id":1266},"what-you-receive","What you receive",[577,1269,1270,1271,942],{},"The keep hook gets a ",[592,1272,1273],{},"TailSamplingContext",[720,1275,1277],{"className":722,"code":1276,"language":725,"meta":726,"style":726},"interface TailSamplingContext {\n  \u002F** The event level (debug | info | warn | error) *\u002F\n  level: string\n  \u002F** HTTP response status, if known *\u002F\n  status?: number\n  \u002F** Request duration in milliseconds, if measured *\u002F\n  duration?: number\n  \u002F** The full accumulated context (everything log.set'd) *\u002F\n  context: Record\u003Cstring, unknown>\n  \u002F** The fully enriched event ready to drain *\u002F\n  event: WideEvent\n  \u002F** Mutable: set to true to force-keep this event *\u002F\n  shouldKeep: boolean\n}\n",[592,1278,1279,1290,1295,1305,1310,1321,1326,1335,1340,1364,1369,1379,1384,1394],{"__ignoreMap":726},[730,1280,1281,1284,1288],{"class":732,"line":733},[730,1282,1283],{"class":777},"interface",[730,1285,1287],{"class":1286},"sBMFI"," TailSamplingContext",[730,1289,781],{"class":740},[730,1291,1292],{"class":732,"line":784},[730,1293,1294],{"class":1055},"  \u002F** The event level (debug | info | warn | error) *\u002F\n",[730,1296,1297,1300,1302],{"class":732,"line":847},[730,1298,1299],{"class":791},"  level",[730,1301,942],{"class":740},[730,1303,1304],{"class":1286}," string\n",[730,1306,1307],{"class":732,"line":865},[730,1308,1309],{"class":1055},"  \u002F** HTTP response status, if known *\u002F\n",[730,1311,1312,1315,1318],{"class":732,"line":871},[730,1313,1314],{"class":791},"  status",[730,1316,1317],{"class":740},"?:",[730,1319,1320],{"class":1286}," number\n",[730,1322,1323],{"class":732,"line":968},[730,1324,1325],{"class":1055},"  \u002F** Request duration in milliseconds, if measured *\u002F\n",[730,1327,1328,1331,1333],{"class":732,"line":1014},[730,1329,1330],{"class":791},"  duration",[730,1332,1317],{"class":740},[730,1334,1320],{"class":1286},[730,1336,1337],{"class":732,"line":1028},[730,1338,1339],{"class":1055},"  \u002F** The full accumulated context (everything log.set'd) *\u002F\n",[730,1341,1342,1345,1347,1350,1353,1356,1358,1361],{"class":732,"line":1034},[730,1343,1344],{"class":791},"  context",[730,1346,942],{"class":740},[730,1348,1349],{"class":1286}," Record",[730,1351,1352],{"class":740},"\u003C",[730,1354,1355],{"class":1286},"string",[730,1357,764],{"class":740},[730,1359,1360],{"class":1286}," unknown",[730,1362,1363],{"class":740},">\n",[730,1365,1366],{"class":732,"line":1040},[730,1367,1368],{"class":1055},"  \u002F** The fully enriched event ready to drain *\u002F\n",[730,1370,1371,1374,1376],{"class":732,"line":1047},[730,1372,1373],{"class":791},"  event",[730,1375,942],{"class":740},[730,1377,1378],{"class":1286}," WideEvent\n",[730,1380,1381],{"class":732,"line":1052},[730,1382,1383],{"class":1055},"  \u002F** Mutable: set to true to force-keep this event *\u002F\n",[730,1385,1386,1389,1391],{"class":732,"line":1059},[730,1387,1388],{"class":791},"  shouldKeep",[730,1390,942],{"class":740},[730,1392,1393],{"class":1286}," boolean\n",[730,1395,1397],{"class":732,"line":1396},14,[730,1398,1399],{"class":740},"}\n",[577,1401,1402,1403,1406,1407,1410],{},"Setting ",[592,1404,1405],{},"shouldKeep = true"," forces the event through. 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