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This page covers the ",[551,576,577],{},"extension surface",": how to plug your own keep logic into the pipeline.",[580,581,584,587,631],"prompt",{":actions":582,"description":583,"icon":64},"[\"copy\",\"cursor\",\"claude\"]","Configure tail sampling on evlog",[548,585,586],{},"Set up tail sampling so I keep all errors and slow requests while dropping healthy noise.",[588,589,590,597,611,614,624],"ul",{},[591,592,593,594],"li",{},"Start with the built-in declarative rules: ",[563,595,596],{},"evlog.sampling.keep = { status: '>=400', duration: '>1000', path: ['\u002Fapi\u002Fauth\u002F*'] }",[591,598,599,600,602,603,606,607,610],{},"For multi-field or derived conditions, register an ",[563,601,569],{}," hook (Nitro: ",[563,604,605],{},"nitroApp.hooks.hook('evlog:emit:keep', (ctx) => ...)","); set ",[563,608,609],{},"ctx.shouldKeep = true"," to keep",[591,612,613],{},"Keep the hook fast. 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Drop to a custom hook when you need:",[588,651,652,666,674],{},[591,653,654,657,658,661,662,665],{},[551,655,656],{},"Conditional logic on more than one field"," (e.g. \"keep if ",[563,659,660],{},"status >= 500"," AND ",[563,663,664],{},"user.plan === 'enterprise'","\")",[591,667,668,657,671,665],{},[551,669,670],{},"Keep based on a derived value",[563,672,673],{},"event.audit?.context.actor.role === 'admin'",[591,675,676,679],{},[551,677,678],{},"Stateful decisions"," (rare; needs care since sampling runs in the hot path)",[640,681,683],{"id":682},"custom-keep-hook","Custom keep hook",[548,685,686],{},"The hook signature is the same regardless of framework. The wiring depends on your runtime.",[688,689,690,849],"code-group",{},[691,692,698],"pre",{"className":693,"code":694,"filename":695,"language":696,"meta":697,"style":697},"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","",[563,699,700,753,816,834,840],{"__ignoreMap":697},[701,702,705,709,713,716,718,722,725,728,731,733,736,739,743,746,750],"span",{"class":703,"line":704},"line",1,[701,706,708],{"class":707},"sTEyZ","nitroApp",[701,710,712],{"class":711},"sMK4o",".",[701,714,715],{"class":707},"hooks",[701,717,712],{"class":711},[701,719,721],{"class":720},"s2Zo4","hook",[701,723,724],{"class":707},"(",[701,726,727],{"class":711},"'",[701,729,569],{"class":730},"sfazB",[701,732,727],{"class":711},[701,734,735],{"class":711},",",[701,737,738],{"class":711}," (",[701,740,742],{"class":741},"sHdIc","ctx",[701,744,745],{"class":711},")",[701,747,749],{"class":748},"spNyl"," =>",[701,751,752],{"class":711}," {\n",[701,754,756,760,763,765,767,770,772,775,778,781,784,787,790,792,795,798,800,803,806,810,813],{"class":703,"line":755},2,[701,757,759],{"class":758},"s7zQu","  if",[701,761,738],{"class":762},"swJcz",[701,764,742],{"class":707},[701,766,712],{"class":711},[701,768,769],{"class":707},"context",[701,771,712],{"class":711},[701,773,774],{"class":707},"user",[701,776,777],{"class":711},"?.",[701,779,780],{"class":707},"plan",[701,782,783],{"class":711}," ===",[701,785,786],{"class":711}," '",[701,788,789],{"class":730},"enterprise",[701,791,727],{"class":711},[701,793,794],{"class":711}," &&",[701,796,797],{"class":707}," ctx",[701,799,712],{"class":711},[701,801,802],{"class":707},"status",[701,804,805],{"class":711}," >=",[701,807,809],{"class":808},"sbssI"," 500",[701,811,812],{"class":762},") ",[701,814,815],{"class":711},"{\n",[701,817,819,822,824,827,830],{"class":703,"line":818},3,[701,820,821],{"class":707},"    ctx",[701,823,712],{"class":711},[701,825,826],{"class":707},"shouldKeep",[701,828,829],{"class":711}," =",[701,831,833],{"class":832},"sfNiH"," true\n",[701,835,837],{"class":703,"line":836},4,[701,838,839],{"class":711},"  }\n",[701,841,843,846],{"class":703,"line":842},5,[701,844,845],{"class":711},"}",[701,847,848],{"class":707},")\n",[691,850,853],{"className":693,"code":851,"filename":852,"language":696,"meta":697,"style":697},"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)",[563,854,855,880,886,906,924,937,983,997,1003,1009,1016,1021,1028],{"__ignoreMap":697},[701,856,857,860,863,866,869,872,874,877],{"class":703,"line":704},[701,858,859],{"class":758},"import",[701,861,862],{"class":711}," {",[701,864,865],{"class":707}," definePlugin",[701,867,868],{"class":711}," }",[701,870,871],{"class":758}," from",[701,873,786],{"class":711},[701,875,876],{"class":730},"evlog\u002Ftoolkit",[701,878,879],{"class":711},"'\n",[701,881,882],{"class":703,"line":755},[701,883,885],{"emptyLinePlaceholder":884},true,"\n",[701,887,888,891,894,897,900,902,904],{"class":703,"line":818},[701,889,890],{"class":758},"export",[701,892,893],{"class":748}," const",[701,895,896],{"class":707}," keepEnterpriseErrors ",[701,898,899],{"class":711},"=",[701,901,865],{"class":720},[701,903,724],{"class":707},[701,905,815],{"class":711},[701,907,908,911,914,916,919,921],{"class":703,"line":836},[701,909,910],{"class":762},"  name",[701,912,913],{"class":711},":",[701,915,786],{"class":711},[701,917,918],{"class":730},"keep-enterprise-errors",[701,920,727],{"class":711},[701,922,923],{"class":711},",\n",[701,925,926,929,931,933,935],{"class":703,"line":842},[701,927,928],{"class":762},"  keep",[701,930,724],{"class":711},[701,932,742],{"class":741},[701,934,745],{"class":711},[701,936,752],{"class":711},[701,938,940,943,945,947,949,951,953,955,957,959,961,963,965,967,969,971,973,975,977,979,981],{"class":703,"line":939},6,[701,941,942],{"class":758},"    if",[701,944,738],{"class":762},[701,946,742],{"class":707},[701,948,712],{"class":711},[701,950,769],{"class":707},[701,952,712],{"class":711},[701,954,774],{"class":707},[701,956,777],{"class":711},[701,958,780],{"class":707},[701,960,783],{"class":711},[701,962,786],{"class":711},[701,964,789],{"class":730},[701,966,727],{"class":711},[701,968,794],{"class":711},[701,970,797],{"class":707},[701,972,712],{"class":711},[701,974,802],{"class":707},[701,976,805],{"class":711},[701,978,809],{"class":808},[701,980,812],{"class":762},[701,982,815],{"class":711},[701,984,986,989,991,993,995],{"class":703,"line":985},7,[701,987,988],{"class":707},"      ctx",[701,990,712],{"class":711},[701,992,826],{"class":707},[701,994,829],{"class":711},[701,996,833],{"class":832},[701,998,1000],{"class":703,"line":999},8,[701,1001,1002],{"class":711},"    }\n",[701,1004,1006],{"class":703,"line":1005},9,[701,1007,1008],{"class":711},"  },\n",[701,1010,1012,1014],{"class":703,"line":1011},10,[701,1013,845],{"class":711},[701,1015,848],{"class":707},[701,1017,1019],{"class":703,"line":1018},11,[701,1020,885],{"emptyLinePlaceholder":884},[701,1022,1024],{"class":703,"line":1023},12,[701,1025,1027],{"class":1026},"sHwdD","\u002F\u002F Then: initLogger({ plugins: [keepEnterpriseErrors] })\n",[701,1029,1031],{"class":703,"line":1030},13,[701,1032,1033],{"class":1026},"\u002F\u002F   or: app.use(evlog({ plugins: [keepEnterpriseErrors] }))\n",[548,1035,1036],{},"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.",[640,1038,1040],{"id":1039},"composing-several-keep-predicates","Composing several keep predicates",[548,1042,1043,1044,1047,1048,1050,1051,1053,1054,1057],{},"Use ",[563,1045,1046],{},"composeKeep"," from ",[563,1049,876],{}," to combine multiple predicates into one hook. Each predicate runs independently and the final ",[563,1052,826],{}," is ",[563,1055,1056],{},"true"," if any of them set it:",[691,1059,1061],{"className":693,"code":1060,"language":696,"meta":697,"style":697},"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",[563,1062,1063,1082,1086,1101,1146,1176,1223],{"__ignoreMap":697},[701,1064,1065,1067,1069,1072,1074,1076,1078,1080],{"class":703,"line":704},[701,1066,859],{"class":758},[701,1068,862],{"class":711},[701,1070,1071],{"class":707}," composeKeep",[701,1073,868],{"class":711},[701,1075,871],{"class":758},[701,1077,786],{"class":711},[701,1079,876],{"class":730},[701,1081,879],{"class":711},[701,1083,1084],{"class":703,"line":755},[701,1085,885],{"emptyLinePlaceholder":884},[701,1087,1088,1091,1094,1096,1098],{"class":703,"line":818},[701,1089,1090],{"class":748},"const",[701,1092,1093],{"class":707}," keep ",[701,1095,899],{"class":711},[701,1097,1071],{"class":720},[701,1099,1100],{"class":707},"([\n",[701,1102,1103,1106,1109,1111,1114,1117,1119,1122,1125,1127,1130,1133,1136,1139,1142,1144],{"class":703,"line":836},[701,1104,1105],{"class":711},"  ({",[701,1107,1108],{"class":741}," duration",[701,1110,735],{"class":711},[701,1112,1113],{"class":741}," shouldKeep",[701,1115,1116],{"class":711}," })",[701,1118,749],{"class":748},[701,1120,1121],{"class":707}," duration ",[701,1123,1124],{"class":711},"&&",[701,1126,1121],{"class":707},[701,1128,1129],{"class":711},">",[701,1131,1132],{"class":808}," 2000",[701,1134,1135],{"class":711}," ?",[701,1137,1138],{"class":832}," true",[701,1140,1141],{"class":711}," :",[701,1143,1113],{"class":707},[701,1145,923],{"class":711},[701,1147,1148,1150,1153,1155,1157,1159,1161,1164,1167,1169,1172,1174],{"class":703,"line":842},[701,1149,1105],{"class":711},[701,1151,1152],{"class":741}," event",[701,1154,1116],{"class":711},[701,1156,749],{"class":748},[701,1158,1152],{"class":707},[701,1160,712],{"class":711},[701,1162,1163],{"class":707},"level ",[701,1165,1166],{"class":711},"===",[701,1168,786],{"class":711},[701,1170,1171],{"class":730},"error",[701,1173,727],{"class":711},[701,1175,923],{"class":711},[701,1177,1178,1180,1183,1185,1188,1190,1192,1194,1196,1198,1200,1203,1205,1207,1209,1211,1213,1216,1219,1221],{"class":703,"line":939},[701,1179,1105],{"class":711},[701,1181,1182],{"class":741}," context",[701,1184,735],{"class":711},[701,1186,1187],{"class":741}," status",[701,1189,1116],{"class":711},[701,1191,749],{"class":748},[701,1193,1182],{"class":707},[701,1195,712],{"class":711},[701,1197,774],{"class":707},[701,1199,777],{"class":711},[701,1201,1202],{"class":707},"plan ",[701,1204,1166],{"class":711},[701,1206,786],{"class":711},[701,1208,789],{"class":730},[701,1210,727],{"class":711},[701,1212,794],{"class":711},[701,1214,1215],{"class":707}," status ",[701,1217,1218],{"class":711},">=",[701,1220,809],{"class":808},[701,1222,923],{"class":711},[701,1224,1225],{"class":703,"line":985},[701,1226,1227],{"class":707},"])\n",[548,1229,1230,1231,1234],{},"Errors in individual predicates are isolated (logged with the ",[563,1232,1233],{},"[evlog\u002Fkeep]"," prefix) so a buggy predicate cannot silently drop legitimate events.",[640,1236,1238],{"id":1237},"what-you-receive","What you receive",[548,1240,1241,1242,913],{},"The keep hook gets a ",[563,1243,1244],{},"TailSamplingContext",[691,1246,1248],{"className":693,"code":1247,"language":696,"meta":697,"style":697},"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",[563,1249,1250,1261,1266,1276,1281,1292,1297,1306,1311,1335,1340,1350,1355,1365],{"__ignoreMap":697},[701,1251,1252,1255,1259],{"class":703,"line":704},[701,1253,1254],{"class":748},"interface",[701,1256,1258],{"class":1257},"sBMFI"," TailSamplingContext",[701,1260,752],{"class":711},[701,1262,1263],{"class":703,"line":755},[701,1264,1265],{"class":1026},"  \u002F** The event level (debug | info | warn | error) *\u002F\n",[701,1267,1268,1271,1273],{"class":703,"line":818},[701,1269,1270],{"class":762},"  level",[701,1272,913],{"class":711},[701,1274,1275],{"class":1257}," string\n",[701,1277,1278],{"class":703,"line":836},[701,1279,1280],{"class":1026},"  \u002F** HTTP response status, if known *\u002F\n",[701,1282,1283,1286,1289],{"class":703,"line":842},[701,1284,1285],{"class":762},"  status",[701,1287,1288],{"class":711},"?:",[701,1290,1291],{"class":1257}," number\n",[701,1293,1294],{"class":703,"line":939},[701,1295,1296],{"class":1026},"  \u002F** Request duration in milliseconds, if measured *\u002F\n",[701,1298,1299,1302,1304],{"class":703,"line":985},[701,1300,1301],{"class":762},"  duration",[701,1303,1288],{"class":711},[701,1305,1291],{"class":1257},[701,1307,1308],{"class":703,"line":999},[701,1309,1310],{"class":1026},"  \u002F** The full accumulated context (everything log.set'd) *\u002F\n",[701,1312,1313,1316,1318,1321,1324,1327,1329,1332],{"class":703,"line":1005},[701,1314,1315],{"class":762},"  context",[701,1317,913],{"class":711},[701,1319,1320],{"class":1257}," Record",[701,1322,1323],{"class":711},"\u003C",[701,1325,1326],{"class":1257},"string",[701,1328,735],{"class":711},[701,1330,1331],{"class":1257}," unknown",[701,1333,1334],{"class":711},">\n",[701,1336,1337],{"class":703,"line":1011},[701,1338,1339],{"class":1026},"  \u002F** The fully enriched event ready to drain *\u002F\n",[701,1341,1342,1345,1347],{"class":703,"line":1018},[701,1343,1344],{"class":762},"  event",[701,1346,913],{"class":711},[701,1348,1349],{"class":1257}," WideEvent\n",[701,1351,1352],{"class":703,"line":1023},[701,1353,1354],{"class":1026},"  \u002F** Mutable: set to true to force-keep this event *\u002F\n",[701,1356,1357,1360,1362],{"class":703,"line":1030},[701,1358,1359],{"class":762},"  shouldKeep",[701,1361,913],{"class":711},[701,1363,1364],{"class":1257}," boolean\n",[701,1366,1368],{"class":703,"line":1367},14,[701,1369,1370],{"class":711},"}\n",[548,1372,1373,1374,1377,1378,1381],{},"Setting ",[563,1375,1376],{},"shouldKeep = true"," forces the event through. Setting ",[563,1379,1380],{},"shouldKeep = false"," is a no-op (other predicates may still keep it; the head sampler decides the default).",[640,1383,1385],{"id":1384},"next-steps","Next steps",[588,1387,1388,1396,1401],{},[591,1389,1390,1392,1393,1395],{},[572,1391,61],{"href":62},": head sampling, tail sampling, the built-in declarative ",[563,1394,565],{}," rules",[591,1397,1398,1400],{},[572,1399,473],{"href":474},": when keep belongs in a multi-hook plugin",[591,1402,1403,1405],{},[572,1404,521],{"href":522},": keep all errors, double-check the rules",[1407,1408,1409],"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":697,"searchDepth":755,"depth":755,"links":1411},[1412,1413,1414,1415,1416],{"id":642,"depth":755,"text":643},{"id":682,"depth":755,"text":683},{"id":1039,"depth":755,"text":1040},{"id":1237,"depth":755,"text":1238},{"id":1384,"depth":755,"text":1385},"Decide after the fact whether to keep an event, knowing its status, duration, and errors. 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