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This page covers the ",[589,614,615],{},"extension surface",": how to plug your own keep logic into the pipeline.",[618,619,622,625,669],"prompt",{":actions":620,"description":621,"icon":69},"[\"copy\",\"cursor\",\"claude\"]","Configure tail sampling on evlog",[586,623,624],{},"Set up tail sampling so I keep all errors and slow requests while dropping healthy noise.",[626,627,628,635,649,652,662],"ul",{},[629,630,631,632],"li",{},"Start with the built-in declarative rules: ",[601,633,634],{},"evlog.sampling.keep = { status: '>=400', duration: '>1000', path: ['\u002Fapi\u002Fauth\u002F*'] }",[629,636,637,638,640,641,644,645,648],{},"For multi-field or derived conditions, register an ",[601,639,607],{}," hook (Nitro: ",[601,642,643],{},"nitroApp.hooks.hook('evlog:emit:keep', (ctx) => ...)","); set ",[601,646,647],{},"ctx.shouldKeep = true"," to keep",[629,650,651],{},"Keep the hook fast. It runs on every request after enrichment, so no I\u002FO and no async work",[629,653,654,655,658,659,661],{},"Combine with head sampling (e.g. 10% of healthy traffic) by setting both ",[601,656,657],{},"sample"," (head) and ",[601,660,603],{}," (tail)",[629,663,664,665,668],{},"Always keep error events (",[601,666,667],{},"level: 'error'",") regardless of sampling; double-check rules don't accidentally drop them",[586,670,671,672],{},"Docs: ",[610,673,674],{"href":674,"rel":675},"https:\u002F\u002Fwww.evlog.dev\u002Fextend\u002Ftail-sampling",[676],"nofollow",[678,679,681],"h2",{"id":680},"when-the-built-in-rules-arent-enough","When the built-in rules aren't enough",[586,683,684,685,687],{},"The built-in declarative ",[601,686,603],{}," rules cover the typical cases (status code thresholds, duration thresholds, path matching, level matching). Drop to a custom hook when you need:",[626,689,690,704,712],{},[629,691,692,695,696,699,700,703],{},[589,693,694],{},"Conditional logic on more than one field"," (e.g. \"keep if ",[601,697,698],{},"status >= 500"," AND ",[601,701,702],{},"user.plan === 'enterprise'","\")",[629,705,706,695,709,703],{},[589,707,708],{},"Keep based on a derived value",[601,710,711],{},"event.audit?.context.actor.role === 'admin'",[629,713,714,717],{},[589,715,716],{},"Stateful decisions"," (rare; needs care since sampling runs in the hot path)",[678,719,721],{"id":720},"custom-keep-hook","Custom keep hook",[586,723,724],{},"The hook signature is the same regardless of framework. The wiring depends on your runtime.",[726,727,728,887],"code-group",{},[729,730,736],"pre",{"className":731,"code":732,"filename":733,"language":734,"meta":735,"style":735},"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","",[601,737,738,791,854,872,878],{"__ignoreMap":735},[739,740,743,747,751,754,756,760,763,766,769,771,774,777,781,784,788],"span",{"class":741,"line":742},"line",1,[739,744,746],{"class":745},"sTEyZ","nitroApp",[739,748,750],{"class":749},"sMK4o",".",[739,752,753],{"class":745},"hooks",[739,755,750],{"class":749},[739,757,759],{"class":758},"s2Zo4","hook",[739,761,762],{"class":745},"(",[739,764,765],{"class":749},"'",[739,767,607],{"class":768},"sfazB",[739,770,765],{"class":749},[739,772,773],{"class":749},",",[739,775,776],{"class":749}," (",[739,778,780],{"class":779},"sHdIc","ctx",[739,782,783],{"class":749},")",[739,785,787],{"class":786},"spNyl"," =>",[739,789,790],{"class":749}," {\n",[739,792,794,798,801,803,805,808,810,813,816,819,822,825,828,830,833,836,838,841,844,848,851],{"class":741,"line":793},2,[739,795,797],{"class":796},"s7zQu","  if",[739,799,776],{"class":800},"swJcz",[739,802,780],{"class":745},[739,804,750],{"class":749},[739,806,807],{"class":745},"context",[739,809,750],{"class":749},[739,811,812],{"class":745},"user",[739,814,815],{"class":749},"?.",[739,817,818],{"class":745},"plan",[739,820,821],{"class":749}," ===",[739,823,824],{"class":749}," '",[739,826,827],{"class":768},"enterprise",[739,829,765],{"class":749},[739,831,832],{"class":749}," &&",[739,834,835],{"class":745}," ctx",[739,837,750],{"class":749},[739,839,840],{"class":745},"status",[739,842,843],{"class":749}," >=",[739,845,847],{"class":846},"sbssI"," 500",[739,849,850],{"class":800},") ",[739,852,853],{"class":749},"{\n",[739,855,857,860,862,865,868],{"class":741,"line":856},3,[739,858,859],{"class":745},"    ctx",[739,861,750],{"class":749},[739,863,864],{"class":745},"shouldKeep",[739,866,867],{"class":749}," =",[739,869,871],{"class":870},"sfNiH"," true\n",[739,873,875],{"class":741,"line":874},4,[739,876,877],{"class":749},"  }\n",[739,879,881,884],{"class":741,"line":880},5,[739,882,883],{"class":749},"}",[739,885,886],{"class":745},")\n",[729,888,891],{"className":731,"code":889,"filename":890,"language":734,"meta":735,"style":735},"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)",[601,892,893,918,924,944,962,975,1021,1035,1041,1047,1054,1059,1066],{"__ignoreMap":735},[739,894,895,898,901,904,907,910,912,915],{"class":741,"line":742},[739,896,897],{"class":796},"import",[739,899,900],{"class":749}," {",[739,902,903],{"class":745}," definePlugin",[739,905,906],{"class":749}," }",[739,908,909],{"class":796}," from",[739,911,824],{"class":749},[739,913,914],{"class":768},"evlog\u002Ftoolkit",[739,916,917],{"class":749},"'\n",[739,919,920],{"class":741,"line":793},[739,921,923],{"emptyLinePlaceholder":922},true,"\n",[739,925,926,929,932,935,938,940,942],{"class":741,"line":856},[739,927,928],{"class":796},"export",[739,930,931],{"class":786}," const",[739,933,934],{"class":745}," keepEnterpriseErrors ",[739,936,937],{"class":749},"=",[739,939,903],{"class":758},[739,941,762],{"class":745},[739,943,853],{"class":749},[739,945,946,949,952,954,957,959],{"class":741,"line":874},[739,947,948],{"class":800},"  name",[739,950,951],{"class":749},":",[739,953,824],{"class":749},[739,955,956],{"class":768},"keep-enterprise-errors",[739,958,765],{"class":749},[739,960,961],{"class":749},",\n",[739,963,964,967,969,971,973],{"class":741,"line":880},[739,965,966],{"class":800},"  keep",[739,968,762],{"class":749},[739,970,780],{"class":779},[739,972,783],{"class":749},[739,974,790],{"class":749},[739,976,978,981,983,985,987,989,991,993,995,997,999,1001,1003,1005,1007,1009,1011,1013,1015,1017,1019],{"class":741,"line":977},6,[739,979,980],{"class":796},"    if",[739,982,776],{"class":800},[739,984,780],{"class":745},[739,986,750],{"class":749},[739,988,807],{"class":745},[739,990,750],{"class":749},[739,992,812],{"class":745},[739,994,815],{"class":749},[739,996,818],{"class":745},[739,998,821],{"class":749},[739,1000,824],{"class":749},[739,1002,827],{"class":768},[739,1004,765],{"class":749},[739,1006,832],{"class":749},[739,1008,835],{"class":745},[739,1010,750],{"class":749},[739,1012,840],{"class":745},[739,1014,843],{"class":749},[739,1016,847],{"class":846},[739,1018,850],{"class":800},[739,1020,853],{"class":749},[739,1022,1024,1027,1029,1031,1033],{"class":741,"line":1023},7,[739,1025,1026],{"class":745},"      ctx",[739,1028,750],{"class":749},[739,1030,864],{"class":745},[739,1032,867],{"class":749},[739,1034,871],{"class":870},[739,1036,1038],{"class":741,"line":1037},8,[739,1039,1040],{"class":749},"    }\n",[739,1042,1044],{"class":741,"line":1043},9,[739,1045,1046],{"class":749},"  },\n",[739,1048,1050,1052],{"class":741,"line":1049},10,[739,1051,883],{"class":749},[739,1053,886],{"class":745},[739,1055,1057],{"class":741,"line":1056},11,[739,1058,923],{"emptyLinePlaceholder":922},[739,1060,1062],{"class":741,"line":1061},12,[739,1063,1065],{"class":1064},"sHwdD","\u002F\u002F Then: initLogger({ plugins: [keepEnterpriseErrors] })\n",[739,1067,1069],{"class":741,"line":1068},13,[739,1070,1071],{"class":1064},"\u002F\u002F   or: app.use(evlog({ plugins: [keepEnterpriseErrors] }))\n",[586,1073,1074],{},"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.",[678,1076,1078],{"id":1077},"composing-several-keep-predicates","Composing several keep predicates",[586,1080,1081,1082,1085,1086,1088,1089,1091,1092,1095],{},"Use ",[601,1083,1084],{},"composeKeep"," from ",[601,1087,914],{}," to combine multiple predicates into one hook. Each predicate runs independently and the final ",[601,1090,864],{}," is ",[601,1093,1094],{},"true"," if any of them set it:",[729,1097,1099],{"className":731,"code":1098,"language":734,"meta":735,"style":735},"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",[601,1100,1101,1120,1124,1139,1184,1214,1261],{"__ignoreMap":735},[739,1102,1103,1105,1107,1110,1112,1114,1116,1118],{"class":741,"line":742},[739,1104,897],{"class":796},[739,1106,900],{"class":749},[739,1108,1109],{"class":745}," composeKeep",[739,1111,906],{"class":749},[739,1113,909],{"class":796},[739,1115,824],{"class":749},[739,1117,914],{"class":768},[739,1119,917],{"class":749},[739,1121,1122],{"class":741,"line":793},[739,1123,923],{"emptyLinePlaceholder":922},[739,1125,1126,1129,1132,1134,1136],{"class":741,"line":856},[739,1127,1128],{"class":786},"const",[739,1130,1131],{"class":745}," keep ",[739,1133,937],{"class":749},[739,1135,1109],{"class":758},[739,1137,1138],{"class":745},"([\n",[739,1140,1141,1144,1147,1149,1152,1155,1157,1160,1163,1165,1168,1171,1174,1177,1180,1182],{"class":741,"line":874},[739,1142,1143],{"class":749},"  ({",[739,1145,1146],{"class":779}," duration",[739,1148,773],{"class":749},[739,1150,1151],{"class":779}," shouldKeep",[739,1153,1154],{"class":749}," })",[739,1156,787],{"class":786},[739,1158,1159],{"class":745}," duration ",[739,1161,1162],{"class":749},"&&",[739,1164,1159],{"class":745},[739,1166,1167],{"class":749},">",[739,1169,1170],{"class":846}," 2000",[739,1172,1173],{"class":749}," ?",[739,1175,1176],{"class":870}," true",[739,1178,1179],{"class":749}," :",[739,1181,1151],{"class":745},[739,1183,961],{"class":749},[739,1185,1186,1188,1191,1193,1195,1197,1199,1202,1205,1207,1210,1212],{"class":741,"line":880},[739,1187,1143],{"class":749},[739,1189,1190],{"class":779}," event",[739,1192,1154],{"class":749},[739,1194,787],{"class":786},[739,1196,1190],{"class":745},[739,1198,750],{"class":749},[739,1200,1201],{"class":745},"level ",[739,1203,1204],{"class":749},"===",[739,1206,824],{"class":749},[739,1208,1209],{"class":768},"error",[739,1211,765],{"class":749},[739,1213,961],{"class":749},[739,1215,1216,1218,1221,1223,1226,1228,1230,1232,1234,1236,1238,1241,1243,1245,1247,1249,1251,1254,1257,1259],{"class":741,"line":977},[739,1217,1143],{"class":749},[739,1219,1220],{"class":779}," context",[739,1222,773],{"class":749},[739,1224,1225],{"class":779}," status",[739,1227,1154],{"class":749},[739,1229,787],{"class":786},[739,1231,1220],{"class":745},[739,1233,750],{"class":749},[739,1235,812],{"class":745},[739,1237,815],{"class":749},[739,1239,1240],{"class":745},"plan ",[739,1242,1204],{"class":749},[739,1244,824],{"class":749},[739,1246,827],{"class":768},[739,1248,765],{"class":749},[739,1250,832],{"class":749},[739,1252,1253],{"class":745}," status ",[739,1255,1256],{"class":749},">=",[739,1258,847],{"class":846},[739,1260,961],{"class":749},[739,1262,1263],{"class":741,"line":1023},[739,1264,1265],{"class":745},"])\n",[586,1267,1268,1269,1272],{},"Errors in individual predicates are isolated (logged with the ",[601,1270,1271],{},"[evlog\u002Fkeep]"," prefix) so a buggy predicate cannot silently drop legitimate events.",[678,1274,1276],{"id":1275},"what-you-receive","What you receive",[586,1278,1279,1280,951],{},"The keep hook gets a ",[601,1281,1282],{},"TailSamplingContext",[729,1284,1286],{"className":731,"code":1285,"language":734,"meta":735,"style":735},"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",[601,1287,1288,1299,1304,1314,1319,1330,1335,1344,1349,1373,1378,1388,1393,1403],{"__ignoreMap":735},[739,1289,1290,1293,1297],{"class":741,"line":742},[739,1291,1292],{"class":786},"interface",[739,1294,1296],{"class":1295},"sBMFI"," TailSamplingContext",[739,1298,790],{"class":749},[739,1300,1301],{"class":741,"line":793},[739,1302,1303],{"class":1064},"  \u002F** The event level (debug | info | warn | error) *\u002F\n",[739,1305,1306,1309,1311],{"class":741,"line":856},[739,1307,1308],{"class":800},"  level",[739,1310,951],{"class":749},[739,1312,1313],{"class":1295}," string\n",[739,1315,1316],{"class":741,"line":874},[739,1317,1318],{"class":1064},"  \u002F** HTTP response status, if known *\u002F\n",[739,1320,1321,1324,1327],{"class":741,"line":880},[739,1322,1323],{"class":800},"  status",[739,1325,1326],{"class":749},"?:",[739,1328,1329],{"class":1295}," number\n",[739,1331,1332],{"class":741,"line":977},[739,1333,1334],{"class":1064},"  \u002F** Request duration in milliseconds, if measured *\u002F\n",[739,1336,1337,1340,1342],{"class":741,"line":1023},[739,1338,1339],{"class":800},"  duration",[739,1341,1326],{"class":749},[739,1343,1329],{"class":1295},[739,1345,1346],{"class":741,"line":1037},[739,1347,1348],{"class":1064},"  \u002F** The full accumulated context (everything log.set'd) *\u002F\n",[739,1350,1351,1354,1356,1359,1362,1365,1367,1370],{"class":741,"line":1043},[739,1352,1353],{"class":800},"  context",[739,1355,951],{"class":749},[739,1357,1358],{"class":1295}," Record",[739,1360,1361],{"class":749},"\u003C",[739,1363,1364],{"class":1295},"string",[739,1366,773],{"class":749},[739,1368,1369],{"class":1295}," unknown",[739,1371,1372],{"class":749},">\n",[739,1374,1375],{"class":741,"line":1049},[739,1376,1377],{"class":1064},"  \u002F** The fully enriched event ready to drain *\u002F\n",[739,1379,1380,1383,1385],{"class":741,"line":1056},[739,1381,1382],{"class":800},"  event",[739,1384,951],{"class":749},[739,1386,1387],{"class":1295}," WideEvent\n",[739,1389,1390],{"class":741,"line":1061},[739,1391,1392],{"class":1064},"  \u002F** Mutable: set to true to force-keep this event *\u002F\n",[739,1394,1395,1398,1400],{"class":741,"line":1068},[739,1396,1397],{"class":800},"  shouldKeep",[739,1399,951],{"class":749},[739,1401,1402],{"class":1295}," boolean\n",[739,1404,1406],{"class":741,"line":1405},14,[739,1407,1408],{"class":749},"}\n",[586,1410,1411,1412,1415,1416,1419],{},"Setting ",[601,1413,1414],{},"shouldKeep = true"," forces the event through. Setting ",[601,1417,1418],{},"shouldKeep = false"," is a no-op (other predicates may still keep it; the head sampler decides the default).",[678,1421,1423],{"id":1422},"next-steps","Next steps",[626,1425,1426,1434,1439],{},[629,1427,1428,1430,1431,1433],{},[610,1429,66],{"href":67},": head sampling, tail sampling, the built-in declarative ",[601,1432,603],{}," rules",[629,1435,1436,1438],{},[610,1437,486],{"href":487},": when keep belongs in a multi-hook plugin",[629,1440,1441,1443],{},[610,1442,534],{"href":535},": keep all errors, double-check the rules",[1445,1446,1447],"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":735,"searchDepth":793,"depth":793,"links":1449},[1450,1451,1452,1453,1454],{"id":680,"depth":793,"text":681},{"id":720,"depth":793,"text":721},{"id":1077,"depth":793,"text":1078},{"id":1275,"depth":793,"text":1276},{"id":1422,"depth":793,"text":1423},"Decide after the fact whether to keep an event, knowing its status, duration, and errors. Keep every error, throw away healthy noise.","md",[1458,1462],{"label":1459,"icon":69,"to":67,"color":1460,"variant":1461},"Sampling Reference","neutral","subtle",{"label":486,"icon":461,"to":487,"color":1460,"variant":1461},{},{"title":494,"icon":69},{"title":578,"description":1455},"xyAHGY07Hgec_LltDe6OXeir6AO7bCpN3ytZL5AkuJs",[1468,1470],{"title":490,"path":491,"stem":492,"description":1469,"icon":443,"children":-1},"Write an enricher that adds deployment metadata, tenant ids, feature flags, or anything computed. Error isolation and merging are handled.",{"title":498,"path":499,"stem":500,"description":1471,"icon":501,"children":-1},"Every drain request carries User-Agent and X-Evlog-Source so receivers can triage the traffic. Override or suppress them when you need to.",1790280248746]