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This page covers the ",[594,619,620],{},"extension surface",": how to plug your own keep logic into the pipeline.",[623,624,627,630,674],"prompt",{":actions":625,"description":626,"icon":69},"[\"copy\",\"cursor\",\"claude\"]","Configure tail sampling on evlog",[591,628,629],{},"Set up tail sampling so I keep all errors and slow requests while dropping healthy noise.",[631,632,633,640,654,657,667],"ul",{},[634,635,636,637],"li",{},"Start with the built-in declarative rules: ",[606,638,639],{},"evlog.sampling.keep = { status: '>=400', duration: '>1000', path: ['\u002Fapi\u002Fauth\u002F*'] }",[634,641,642,643,645,646,649,650,653],{},"For multi-field or derived conditions, register an ",[606,644,612],{}," hook (Nitro: ",[606,647,648],{},"nitroApp.hooks.hook('evlog:emit:keep', (ctx) => ...)","); set ",[606,651,652],{},"ctx.shouldKeep = true"," to keep",[634,655,656],{},"Keep the hook fast. It runs on every request after enrichment, so no I\u002FO and no async work",[634,658,659,660,663,664,666],{},"Combine with head sampling (e.g. 10% of healthy traffic) by setting both ",[606,661,662],{},"sample"," (head) and ",[606,665,608],{}," (tail)",[634,668,669,670,673],{},"Always keep error events (",[606,671,672],{},"level: 'error'",") regardless of sampling; double-check rules don't accidentally drop them",[591,675,676,677],{},"Docs: ",[615,678,679],{"href":679,"rel":680},"https:\u002F\u002Fwww.evlog.dev\u002Fextend\u002Ftail-sampling",[681],"nofollow",[683,684,686],"h2",{"id":685},"when-the-built-in-rules-arent-enough","When the built-in rules aren't enough",[591,688,689,690,692],{},"The built-in declarative ",[606,691,608],{}," rules cover the typical cases (status code thresholds, duration thresholds, path matching, level matching). Drop to a custom hook when you need:",[631,694,695,709,717],{},[634,696,697,700,701,704,705,708],{},[594,698,699],{},"Conditional logic on more than one field"," (e.g. \"keep if ",[606,702,703],{},"status >= 500"," AND ",[606,706,707],{},"user.plan === 'enterprise'","\")",[634,710,711,700,714,708],{},[594,712,713],{},"Keep based on a derived value",[606,715,716],{},"event.audit?.context.actor.role === 'admin'",[634,718,719,722],{},[594,720,721],{},"Stateful decisions"," (rare; needs care since sampling runs in the hot path)",[683,724,726],{"id":725},"custom-keep-hook","Custom keep hook",[591,728,729],{},"The hook signature is the same regardless of framework. The wiring depends on your runtime.",[731,732,733,892],"code-group",{},[734,735,741],"pre",{"className":736,"code":737,"filename":738,"language":739,"meta":740,"style":740},"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","",[606,742,743,796,859,877,883],{"__ignoreMap":740},[744,745,748,752,756,759,761,765,768,771,774,776,779,782,786,789,793],"span",{"class":746,"line":747},"line",1,[744,749,751],{"class":750},"sTEyZ","nitroApp",[744,753,755],{"class":754},"sMK4o",".",[744,757,758],{"class":750},"hooks",[744,760,755],{"class":754},[744,762,764],{"class":763},"s2Zo4","hook",[744,766,767],{"class":750},"(",[744,769,770],{"class":754},"'",[744,772,612],{"class":773},"sfazB",[744,775,770],{"class":754},[744,777,778],{"class":754},",",[744,780,781],{"class":754}," (",[744,783,785],{"class":784},"sHdIc","ctx",[744,787,788],{"class":754},")",[744,790,792],{"class":791},"spNyl"," =>",[744,794,795],{"class":754}," {\n",[744,797,799,803,806,808,810,813,815,818,821,824,827,830,833,835,838,841,843,846,849,853,856],{"class":746,"line":798},2,[744,800,802],{"class":801},"s7zQu","  if",[744,804,781],{"class":805},"swJcz",[744,807,785],{"class":750},[744,809,755],{"class":754},[744,811,812],{"class":750},"context",[744,814,755],{"class":754},[744,816,817],{"class":750},"user",[744,819,820],{"class":754},"?.",[744,822,823],{"class":750},"plan",[744,825,826],{"class":754}," ===",[744,828,829],{"class":754}," '",[744,831,832],{"class":773},"enterprise",[744,834,770],{"class":754},[744,836,837],{"class":754}," &&",[744,839,840],{"class":750}," ctx",[744,842,755],{"class":754},[744,844,845],{"class":750},"status",[744,847,848],{"class":754}," >=",[744,850,852],{"class":851},"sbssI"," 500",[744,854,855],{"class":805},") ",[744,857,858],{"class":754},"{\n",[744,860,862,865,867,870,873],{"class":746,"line":861},3,[744,863,864],{"class":750},"    ctx",[744,866,755],{"class":754},[744,868,869],{"class":750},"shouldKeep",[744,871,872],{"class":754}," =",[744,874,876],{"class":875},"sfNiH"," true\n",[744,878,880],{"class":746,"line":879},4,[744,881,882],{"class":754},"  }\n",[744,884,886,889],{"class":746,"line":885},5,[744,887,888],{"class":754},"}",[744,890,891],{"class":750},")\n",[734,893,896],{"className":736,"code":894,"filename":895,"language":739,"meta":740,"style":740},"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)",[606,897,898,923,929,949,967,980,1026,1040,1046,1052,1059,1064,1071],{"__ignoreMap":740},[744,899,900,903,906,909,912,915,917,920],{"class":746,"line":747},[744,901,902],{"class":801},"import",[744,904,905],{"class":754}," {",[744,907,908],{"class":750}," definePlugin",[744,910,911],{"class":754}," }",[744,913,914],{"class":801}," from",[744,916,829],{"class":754},[744,918,919],{"class":773},"evlog\u002Ftoolkit",[744,921,922],{"class":754},"'\n",[744,924,925],{"class":746,"line":798},[744,926,928],{"emptyLinePlaceholder":927},true,"\n",[744,930,931,934,937,940,943,945,947],{"class":746,"line":861},[744,932,933],{"class":801},"export",[744,935,936],{"class":791}," const",[744,938,939],{"class":750}," keepEnterpriseErrors ",[744,941,942],{"class":754},"=",[744,944,908],{"class":763},[744,946,767],{"class":750},[744,948,858],{"class":754},[744,950,951,954,957,959,962,964],{"class":746,"line":879},[744,952,953],{"class":805},"  name",[744,955,956],{"class":754},":",[744,958,829],{"class":754},[744,960,961],{"class":773},"keep-enterprise-errors",[744,963,770],{"class":754},[744,965,966],{"class":754},",\n",[744,968,969,972,974,976,978],{"class":746,"line":885},[744,970,971],{"class":805},"  keep",[744,973,767],{"class":754},[744,975,785],{"class":784},[744,977,788],{"class":754},[744,979,795],{"class":754},[744,981,983,986,988,990,992,994,996,998,1000,1002,1004,1006,1008,1010,1012,1014,1016,1018,1020,1022,1024],{"class":746,"line":982},6,[744,984,985],{"class":801},"    if",[744,987,781],{"class":805},[744,989,785],{"class":750},[744,991,755],{"class":754},[744,993,812],{"class":750},[744,995,755],{"class":754},[744,997,817],{"class":750},[744,999,820],{"class":754},[744,1001,823],{"class":750},[744,1003,826],{"class":754},[744,1005,829],{"class":754},[744,1007,832],{"class":773},[744,1009,770],{"class":754},[744,1011,837],{"class":754},[744,1013,840],{"class":750},[744,1015,755],{"class":754},[744,1017,845],{"class":750},[744,1019,848],{"class":754},[744,1021,852],{"class":851},[744,1023,855],{"class":805},[744,1025,858],{"class":754},[744,1027,1029,1032,1034,1036,1038],{"class":746,"line":1028},7,[744,1030,1031],{"class":750},"      ctx",[744,1033,755],{"class":754},[744,1035,869],{"class":750},[744,1037,872],{"class":754},[744,1039,876],{"class":875},[744,1041,1043],{"class":746,"line":1042},8,[744,1044,1045],{"class":754},"    }\n",[744,1047,1049],{"class":746,"line":1048},9,[744,1050,1051],{"class":754},"  },\n",[744,1053,1055,1057],{"class":746,"line":1054},10,[744,1056,888],{"class":754},[744,1058,891],{"class":750},[744,1060,1062],{"class":746,"line":1061},11,[744,1063,928],{"emptyLinePlaceholder":927},[744,1065,1067],{"class":746,"line":1066},12,[744,1068,1070],{"class":1069},"sHwdD","\u002F\u002F Then: initLogger({ plugins: [keepEnterpriseErrors] })\n",[744,1072,1074],{"class":746,"line":1073},13,[744,1075,1076],{"class":1069},"\u002F\u002F   or: app.use(evlog({ plugins: [keepEnterpriseErrors] }))\n",[591,1078,1079],{},"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.",[683,1081,1083],{"id":1082},"composing-several-keep-predicates","Composing several keep predicates",[591,1085,1086,1087,1090,1091,1093,1094,1096,1097,1100],{},"Use ",[606,1088,1089],{},"composeKeep"," from ",[606,1092,919],{}," to combine multiple predicates into one hook. Each predicate runs independently and the final ",[606,1095,869],{}," is ",[606,1098,1099],{},"true"," if any of them set it:",[734,1102,1104],{"className":736,"code":1103,"language":739,"meta":740,"style":740},"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",[606,1105,1106,1125,1129,1144,1189,1219,1266],{"__ignoreMap":740},[744,1107,1108,1110,1112,1115,1117,1119,1121,1123],{"class":746,"line":747},[744,1109,902],{"class":801},[744,1111,905],{"class":754},[744,1113,1114],{"class":750}," composeKeep",[744,1116,911],{"class":754},[744,1118,914],{"class":801},[744,1120,829],{"class":754},[744,1122,919],{"class":773},[744,1124,922],{"class":754},[744,1126,1127],{"class":746,"line":798},[744,1128,928],{"emptyLinePlaceholder":927},[744,1130,1131,1134,1137,1139,1141],{"class":746,"line":861},[744,1132,1133],{"class":791},"const",[744,1135,1136],{"class":750}," keep ",[744,1138,942],{"class":754},[744,1140,1114],{"class":763},[744,1142,1143],{"class":750},"([\n",[744,1145,1146,1149,1152,1154,1157,1160,1162,1165,1168,1170,1173,1176,1179,1182,1185,1187],{"class":746,"line":879},[744,1147,1148],{"class":754},"  ({",[744,1150,1151],{"class":784}," duration",[744,1153,778],{"class":754},[744,1155,1156],{"class":784}," shouldKeep",[744,1158,1159],{"class":754}," })",[744,1161,792],{"class":791},[744,1163,1164],{"class":750}," duration ",[744,1166,1167],{"class":754},"&&",[744,1169,1164],{"class":750},[744,1171,1172],{"class":754},">",[744,1174,1175],{"class":851}," 2000",[744,1177,1178],{"class":754}," ?",[744,1180,1181],{"class":875}," true",[744,1183,1184],{"class":754}," :",[744,1186,1156],{"class":750},[744,1188,966],{"class":754},[744,1190,1191,1193,1196,1198,1200,1202,1204,1207,1210,1212,1215,1217],{"class":746,"line":885},[744,1192,1148],{"class":754},[744,1194,1195],{"class":784}," event",[744,1197,1159],{"class":754},[744,1199,792],{"class":791},[744,1201,1195],{"class":750},[744,1203,755],{"class":754},[744,1205,1206],{"class":750},"level ",[744,1208,1209],{"class":754},"===",[744,1211,829],{"class":754},[744,1213,1214],{"class":773},"error",[744,1216,770],{"class":754},[744,1218,966],{"class":754},[744,1220,1221,1223,1226,1228,1231,1233,1235,1237,1239,1241,1243,1246,1248,1250,1252,1254,1256,1259,1262,1264],{"class":746,"line":982},[744,1222,1148],{"class":754},[744,1224,1225],{"class":784}," context",[744,1227,778],{"class":754},[744,1229,1230],{"class":784}," status",[744,1232,1159],{"class":754},[744,1234,792],{"class":791},[744,1236,1225],{"class":750},[744,1238,755],{"class":754},[744,1240,817],{"class":750},[744,1242,820],{"class":754},[744,1244,1245],{"class":750},"plan ",[744,1247,1209],{"class":754},[744,1249,829],{"class":754},[744,1251,832],{"class":773},[744,1253,770],{"class":754},[744,1255,837],{"class":754},[744,1257,1258],{"class":750}," status ",[744,1260,1261],{"class":754},">=",[744,1263,852],{"class":851},[744,1265,966],{"class":754},[744,1267,1268],{"class":746,"line":1028},[744,1269,1270],{"class":750},"])\n",[591,1272,1273,1274,1277],{},"Errors in individual predicates are isolated (logged with the ",[606,1275,1276],{},"[evlog\u002Fkeep]"," prefix) so a buggy predicate cannot silently drop legitimate events.",[683,1279,1281],{"id":1280},"what-you-receive","What you receive",[591,1283,1284,1285,956],{},"The keep hook gets a ",[606,1286,1287],{},"TailSamplingContext",[734,1289,1291],{"className":736,"code":1290,"language":739,"meta":740,"style":740},"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",[606,1292,1293,1304,1309,1319,1324,1335,1340,1349,1354,1378,1383,1393,1398,1408],{"__ignoreMap":740},[744,1294,1295,1298,1302],{"class":746,"line":747},[744,1296,1297],{"class":791},"interface",[744,1299,1301],{"class":1300},"sBMFI"," TailSamplingContext",[744,1303,795],{"class":754},[744,1305,1306],{"class":746,"line":798},[744,1307,1308],{"class":1069},"  \u002F** The event level (debug | info | warn | error) *\u002F\n",[744,1310,1311,1314,1316],{"class":746,"line":861},[744,1312,1313],{"class":805},"  level",[744,1315,956],{"class":754},[744,1317,1318],{"class":1300}," string\n",[744,1320,1321],{"class":746,"line":879},[744,1322,1323],{"class":1069},"  \u002F** HTTP response status, if known *\u002F\n",[744,1325,1326,1329,1332],{"class":746,"line":885},[744,1327,1328],{"class":805},"  status",[744,1330,1331],{"class":754},"?:",[744,1333,1334],{"class":1300}," number\n",[744,1336,1337],{"class":746,"line":982},[744,1338,1339],{"class":1069},"  \u002F** Request duration in milliseconds, if measured *\u002F\n",[744,1341,1342,1345,1347],{"class":746,"line":1028},[744,1343,1344],{"class":805},"  duration",[744,1346,1331],{"class":754},[744,1348,1334],{"class":1300},[744,1350,1351],{"class":746,"line":1042},[744,1352,1353],{"class":1069},"  \u002F** The full accumulated context (everything log.set'd) *\u002F\n",[744,1355,1356,1359,1361,1364,1367,1370,1372,1375],{"class":746,"line":1048},[744,1357,1358],{"class":805},"  context",[744,1360,956],{"class":754},[744,1362,1363],{"class":1300}," Record",[744,1365,1366],{"class":754},"\u003C",[744,1368,1369],{"class":1300},"string",[744,1371,778],{"class":754},[744,1373,1374],{"class":1300}," unknown",[744,1376,1377],{"class":754},">\n",[744,1379,1380],{"class":746,"line":1054},[744,1381,1382],{"class":1069},"  \u002F** The fully enriched event ready to drain *\u002F\n",[744,1384,1385,1388,1390],{"class":746,"line":1061},[744,1386,1387],{"class":805},"  event",[744,1389,956],{"class":754},[744,1391,1392],{"class":1300}," WideEvent\n",[744,1394,1395],{"class":746,"line":1066},[744,1396,1397],{"class":1069},"  \u002F** Mutable: set to true to force-keep this event *\u002F\n",[744,1399,1400,1403,1405],{"class":746,"line":1073},[744,1401,1402],{"class":805},"  shouldKeep",[744,1404,956],{"class":754},[744,1406,1407],{"class":1300}," boolean\n",[744,1409,1411],{"class":746,"line":1410},14,[744,1412,1413],{"class":754},"}\n",[591,1415,1416,1417,1420,1421,1424],{},"Setting ",[606,1418,1419],{},"shouldKeep = true"," forces the event through. Setting ",[606,1422,1423],{},"shouldKeep = false"," is a no-op (other predicates may still keep it; the head sampler decides the default).",[683,1426,1428],{"id":1427},"next-steps","Next steps",[631,1430,1431,1439,1444],{},[634,1432,1433,1435,1436,1438],{},[615,1434,66],{"href":67},": head sampling, tail sampling, the built-in declarative ",[606,1437,608],{}," rules",[634,1440,1441,1443],{},[615,1442,491],{"href":492},": when keep belongs in a multi-hook plugin",[634,1445,1446,1448],{},[615,1447,539],{"href":540},": keep all errors, double-check the rules",[1450,1451,1452],"style",{},"html pre.shiki code .sTEyZ, html code.shiki .sTEyZ{--shiki-light:#90A4AE;--shiki-default:#EEFFFF;--shiki-dark:#BABED8}html pre.shiki code .sMK4o, html code.shiki .sMK4o{--shiki-light:#39ADB5;--shiki-default:#89DDFF;--shiki-dark:#89DDFF}html pre.shiki code .s2Zo4, html code.shiki .s2Zo4{--shiki-light:#6182B8;--shiki-default:#82AAFF;--shiki-dark:#82AAFF}html pre.shiki code .sfazB, html code.shiki .sfazB{--shiki-light:#91B859;--shiki-default:#C3E88D;--shiki-dark:#C3E88D}html pre.shiki code .sHdIc, html code.shiki .sHdIc{--shiki-light:#90A4AE;--shiki-light-font-style:italic;--shiki-default:#EEFFFF;--shiki-default-font-style:italic;--shiki-dark:#BABED8;--shiki-dark-font-style:italic}html pre.shiki code .spNyl, html code.shiki .spNyl{--shiki-light:#9C3EDA;--shiki-default:#C792EA;--shiki-dark:#C792EA}html pre.shiki code .s7zQu, html code.shiki .s7zQu{--shiki-light:#39ADB5;--shiki-light-font-style:italic;--shiki-default:#89DDFF;--shiki-default-font-style:italic;--shiki-dark:#89DDFF;--shiki-dark-font-style:italic}html pre.shiki code .swJcz, html code.shiki .swJcz{--shiki-light:#E53935;--shiki-default:#F07178;--shiki-dark:#F07178}html pre.shiki code .sbssI, html code.shiki .sbssI{--shiki-light:#F76D47;--shiki-default:#F78C6C;--shiki-dark:#F78C6C}html pre.shiki code .sfNiH, html code.shiki .sfNiH{--shiki-light:#FF5370;--shiki-default:#FF9CAC;--shiki-dark:#FF9CAC}html .light .shiki span {color: var(--shiki-light);background: var(--shiki-light-bg);font-style: var(--shiki-light-font-style);font-weight: var(--shiki-light-font-weight);text-decoration: var(--shiki-light-text-decoration);}html.light .shiki span {color: var(--shiki-light);background: var(--shiki-light-bg);font-style: var(--shiki-light-font-style);font-weight: var(--shiki-light-font-weight);text-decoration: var(--shiki-light-text-decoration);}html .default .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html .dark .shiki span {color: var(--shiki-dark);background: var(--shiki-dark-bg);font-style: var(--shiki-dark-font-style);font-weight: var(--shiki-dark-font-weight);text-decoration: var(--shiki-dark-text-decoration);}html.dark .shiki span {color: var(--shiki-dark);background: var(--shiki-dark-bg);font-style: var(--shiki-dark-font-style);font-weight: var(--shiki-dark-font-weight);text-decoration: var(--shiki-dark-text-decoration);}html pre.shiki code .sHwdD, html code.shiki .sHwdD{--shiki-light:#90A4AE;--shiki-light-font-style:italic;--shiki-default:#546E7A;--shiki-default-font-style:italic;--shiki-dark:#676E95;--shiki-dark-font-style:italic}html pre.shiki code .sBMFI, html code.shiki .sBMFI{--shiki-light:#E2931D;--shiki-default:#FFCB6B;--shiki-dark:#FFCB6B}",{"title":740,"searchDepth":798,"depth":798,"links":1454},[1455,1456,1457,1458,1459],{"id":685,"depth":798,"text":686},{"id":725,"depth":798,"text":726},{"id":1082,"depth":798,"text":1083},{"id":1280,"depth":798,"text":1281},{"id":1427,"depth":798,"text":1428},"Decide after the fact whether to keep an event, knowing its status, duration, and errors. 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