Reading and combining prices

Reading one oracle means trusting one publisher. A safer habit is to ask several and compare.

Suppose five oracles post a price for the same pair. Four say about 100. One says 900. A plain average gets dragged up to 260 by the odd one. Two better answers:

• The median is the middle value when you line them up. Here it is about 100.
• A trimmed mean throws away the highest and lowest values first, then averages the rest. Also about 100.

The XRP Ledger's servers can do this math for you. One request names a pair and a list of oracles. The answer comes back with the mean, the median and, if you ask, a trimmed mean.

Age matters as much as agreement. A price from an hour ago can be far from the market now. The same request can set a time window. Oracles whose last update falls outside the window are left out.

Before relying on any oracle, look at it:

• Who owns it? Check the owner address, not only the provider name. The name is free text.
• When was it updated? Compare the timestamp with the current time.
• Does the pair have a price? A pair with no number was skipped in the last update.

No setting makes a bad source good. Aggregation limits the damage from one wrong oracle. It does not help if most of the list is wrong.

Reading one oracle. Use ledger_entry with the owner's account and the OracleDocumentID. The response is the Oracle entry with its PriceDataSeries and LastUpdateTime.

get_aggregate_price request:

Response:

• entire_set: mean, size and standard_deviation for all collected prices.
• trimmed_set: the same three values after trimming. Present only when trim was sent.
• median: the median price.
• time: the most recent LastUpdateTime among the oracles, in Unix time.

How the data set is built:

• The newest LastUpdateTime among the listed oracles is the upper time limit. trim_threshold counts back from it.
• An oracle is included if it has the pair, has an AssetPrice and falls inside the time range.
• If an oracle's current entry has no AssetPrice for the pair, the server examines up to three previous versions and uses the most recent one that qualifies.
• Duplicate oracle identifiers are ignored.

Errors:

• objectNotFound: no prices in the data set.
• internal: trim_threshold removed every price.
• oracleMalformed: the oracles array is malformed or outside 1 to 200.

The method is read-only. It runs on a server and returns statistics. It does not write an aggregate price to the ledger.

Related: Prices on the ledger, Running a node: xrpld and Clio.