|
| 1 | +// a faithful reimplementation of https://pkg.go.dev/golang.org/x/time/rate#Limiter |
| 2 | +// using virtual object state |
| 3 | + |
| 4 | +import { object, ObjectContext } from "@restatedev/restate-sdk"; |
| 5 | + |
| 6 | +type LimiterState = { |
| 7 | + state: LimiterStateInner; |
| 8 | +}; |
| 9 | +type LimiterStateInner = { |
| 10 | + limit: number; |
| 11 | + burst: number; |
| 12 | + tokens: number; |
| 13 | + // last is the last time the limiter's tokens field was updated, in unix millis |
| 14 | + last: number; |
| 15 | + // lastEvent is the latest time of a rate-limited event (past or future), in unix millis |
| 16 | + lastEvent: number; |
| 17 | +}; |
| 18 | + |
| 19 | +export interface Reservation { |
| 20 | + ok: boolean; |
| 21 | + tokens: number; |
| 22 | + creationDate: number; |
| 23 | + dateToAct: number; |
| 24 | + // This is the Limit at reservation time, it can change later. |
| 25 | + limit: number; |
| 26 | +} |
| 27 | + |
| 28 | +export const limiter = object({ |
| 29 | + name: "limiter", |
| 30 | + handlers: { |
| 31 | + state: async ( |
| 32 | + ctx: ObjectContext<LimiterState>, |
| 33 | + ): Promise<LimiterStateInner> => { |
| 34 | + return getState(ctx); |
| 35 | + }, |
| 36 | + tokens: async (ctx: ObjectContext<LimiterState>): Promise<number> => { |
| 37 | + // deterministic date not needed, as there is only an output entry |
| 38 | + const tokens = advance(await getState(ctx), Date.now()); |
| 39 | + return tokens; |
| 40 | + }, |
| 41 | + reserve: async ( |
| 42 | + ctx: ObjectContext<LimiterState>, |
| 43 | + { |
| 44 | + n = 1, |
| 45 | + waitLimitMillis = Infinity, |
| 46 | + }: { n?: number; waitLimitMillis?: number }, |
| 47 | + ): Promise<Reservation> => { |
| 48 | + let lim = await getState(ctx); |
| 49 | + |
| 50 | + if (lim.limit == Infinity) { |
| 51 | + // deterministic date is not necessary, as this is part of a response body, which won't be replayed. |
| 52 | + const now = Date.now(); |
| 53 | + return { |
| 54 | + ok: true, |
| 55 | + tokens: n, |
| 56 | + creationDate: now, |
| 57 | + dateToAct: now, |
| 58 | + limit: 0, |
| 59 | + }; |
| 60 | + } |
| 61 | + |
| 62 | + let r: Reservation; |
| 63 | + ({ lim, r } = await ctx.run(() => { |
| 64 | + const now = Date.now(); |
| 65 | + let tokens = advance(lim, now); |
| 66 | + |
| 67 | + // Calculate the remaining number of tokens resulting from the request. |
| 68 | + tokens -= n; |
| 69 | + |
| 70 | + // Calculate the wait duration |
| 71 | + let waitDurationMillis = 0; |
| 72 | + if (tokens < 0) { |
| 73 | + waitDurationMillis = durationFromTokens(lim.limit, -tokens); |
| 74 | + } |
| 75 | + |
| 76 | + // Decide result |
| 77 | + const ok = n <= lim.burst && waitDurationMillis <= waitLimitMillis; |
| 78 | + |
| 79 | + // Prepare reservation |
| 80 | + const r = { |
| 81 | + ok, |
| 82 | + tokens: 0, |
| 83 | + creationDate: now, |
| 84 | + dateToAct: 0, |
| 85 | + limit: lim.limit, |
| 86 | + } satisfies Reservation; |
| 87 | + |
| 88 | + if (ok) { |
| 89 | + r.tokens = n; |
| 90 | + r.dateToAct = now + waitDurationMillis; |
| 91 | + |
| 92 | + // Update state |
| 93 | + lim.last = now; |
| 94 | + lim.tokens = tokens; |
| 95 | + lim.lastEvent = r.dateToAct; |
| 96 | + } |
| 97 | + |
| 98 | + return { lim, r }; |
| 99 | + })); |
| 100 | + |
| 101 | + setState(ctx, lim); |
| 102 | + |
| 103 | + return r; |
| 104 | + }, |
| 105 | + setRate: async ( |
| 106 | + ctx: ObjectContext<LimiterState>, |
| 107 | + { newLimit, newBurst }: { newLimit?: number; newBurst?: number }, |
| 108 | + ) => { |
| 109 | + if (newLimit === undefined && newBurst === undefined) { |
| 110 | + return; |
| 111 | + } |
| 112 | + |
| 113 | + let lim = await getState(ctx); |
| 114 | + |
| 115 | + lim = await ctx.run(() => { |
| 116 | + const now = Date.now(); |
| 117 | + const tokens = advance(lim, now); |
| 118 | + |
| 119 | + lim.last = now; |
| 120 | + lim.tokens = tokens; |
| 121 | + if (newLimit !== undefined) lim.limit = newLimit; |
| 122 | + if (newBurst !== undefined) lim.burst = newBurst; |
| 123 | + |
| 124 | + return lim; |
| 125 | + }); |
| 126 | + |
| 127 | + setState(ctx, lim); |
| 128 | + }, |
| 129 | + cancelReservation: async ( |
| 130 | + ctx: ObjectContext<LimiterState>, |
| 131 | + r: Reservation, |
| 132 | + ) => { |
| 133 | + let lim = await getState(ctx); |
| 134 | + |
| 135 | + lim = await ctx.run(() => { |
| 136 | + const now = Date.now(); |
| 137 | + |
| 138 | + if (lim.limit == Infinity || r.tokens == 0 || r.dateToAct < now) { |
| 139 | + return lim; |
| 140 | + } |
| 141 | + |
| 142 | + // calculate tokens to restore |
| 143 | + // The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved |
| 144 | + // after r was obtained. These tokens should not be restored. |
| 145 | + const restoreTokens = |
| 146 | + r.tokens - tokensFromDuration(r.limit, lim.lastEvent - r.dateToAct); |
| 147 | + if (restoreTokens <= 0) { |
| 148 | + return lim; |
| 149 | + } |
| 150 | + // advance time to now |
| 151 | + let tokens = advance(lim, now); |
| 152 | + // calculate new number of tokens |
| 153 | + tokens += restoreTokens; |
| 154 | + if (tokens > lim.burst) { |
| 155 | + tokens = lim.burst; |
| 156 | + } |
| 157 | + // update state |
| 158 | + lim.last = now; |
| 159 | + lim.tokens = tokens; |
| 160 | + if (r.dateToAct == lim.lastEvent) { |
| 161 | + const prevEvent = |
| 162 | + r.dateToAct + durationFromTokens(r.limit, -r.tokens); |
| 163 | + if (prevEvent >= now) { |
| 164 | + lim.lastEvent = prevEvent; |
| 165 | + } |
| 166 | + } |
| 167 | + |
| 168 | + return lim; |
| 169 | + }); |
| 170 | + |
| 171 | + setState(ctx, lim); |
| 172 | + }, |
| 173 | + }, |
| 174 | +}); |
| 175 | + |
| 176 | +function advance(lim: LimiterStateInner, date: number): number { |
| 177 | + let last = lim.last; |
| 178 | + if (date <= last) { |
| 179 | + last = date; |
| 180 | + } |
| 181 | + |
| 182 | + // Calculate the new number of tokens, due to time that passed. |
| 183 | + const elapsedMillis = date - last; |
| 184 | + const delta = tokensFromDuration(lim.limit, elapsedMillis); |
| 185 | + let tokens = lim.tokens + delta; |
| 186 | + if (tokens > lim.burst) { |
| 187 | + tokens = lim.burst; |
| 188 | + } |
| 189 | + |
| 190 | + return tokens; |
| 191 | +} |
| 192 | + |
| 193 | +async function getState( |
| 194 | + ctx: ObjectContext<LimiterState>, |
| 195 | +): Promise<LimiterStateInner> { |
| 196 | + return ( |
| 197 | + (await ctx.get("state")) ?? { |
| 198 | + limit: 0, |
| 199 | + burst: 0, |
| 200 | + tokens: 0, |
| 201 | + last: 0, |
| 202 | + lastEvent: 0, |
| 203 | + } |
| 204 | + ); |
| 205 | +} |
| 206 | + |
| 207 | +async function setState( |
| 208 | + ctx: ObjectContext<LimiterState>, |
| 209 | + lim: LimiterStateInner, |
| 210 | +) { |
| 211 | + ctx.set("state", lim); |
| 212 | +} |
| 213 | + |
| 214 | +function durationFromTokens(limit: number, tokens: number): number { |
| 215 | + if (limit <= 0) { |
| 216 | + return Infinity; |
| 217 | + } |
| 218 | + |
| 219 | + return (tokens / limit) * 1000; |
| 220 | +} |
| 221 | + |
| 222 | +function tokensFromDuration(limit: number, durationMillis: number): number { |
| 223 | + if (limit <= 0) { |
| 224 | + return 0; |
| 225 | + } |
| 226 | + return (durationMillis / 1000) * limit; |
| 227 | +} |
| 228 | + |
| 229 | +export type Limiter = typeof limiter; |
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