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| 'use strict';
const _ = require('lodash');
const async = require('async');
const url = require('url');
const util = require('./util');
// Takes a function(context, finalizedOutput), and returns a new
// function(context, unfinalizedOutput).
exports.page = (send) => {
return (context, output) => {
if ((output.resource.parameters || []).filter(p =>
(p.name === 'page' || p.name === 'pageSize') &&
p.type === 'query').length === 0) {
return send(context, output.finalize());
}
const errStrategy = createStrategy(output);
const err = errStrategy[0]; const strategy = errStrategy[1];
if (util.notNull(err) || !util.notNull(strategy)) {
return context.fail(util.failure(400,
`Can't determine paging strategy: ${err}`));
}
let objs = [];
async.during(
cb => {
let innerContext = _.assign(_.clone(context), {
succeed: data => strategy.hasMore(objs, data, cb),
fail: failure => cb(failure),
done: (err, data) => {
Iif (util.notNull(err)) { cb(err); }
else { strategy.hasMore(objs, data, cb); }
}
});
let innerOutput = _.cloneDeep(output).finalize(strategy.input());
send(innerContext, innerOutput);
},
cb => cb(), // noop: all the work is done in the test
failure => {
if (util.notNull(failure)) { return context.fail(failure); }
else {
return context.succeed(convertMerged(output, strategy, objs));
}
}
);
};
};
// Find a pagination strategy that lets us iterate through all the
// backend requests to get the data that we need to return.
const createStrategy = (output) => {
const inQuery = output.input.queryParameters || {};
const nextDetails = util.decodeJson64(inQuery.nextPage) || {};
let isCursor = false;
let page = inQuery.page;
if (!util.notNull(page)) {
page = nextDetails.page;
isCursor = true;
} else { page -= 1; }
if (!util.notNull(page)) { page = 0; }
page = Math.max(0, page);
let pageSize = inQuery.pageSize;
if (!util.notNull(pageSize)) { pageSize = nextDetails.pageSize; }
if (!util.notNull(pageSize)) { pageSize = 50; }
pageSize = Math.max(0, pageSize);
nextDetails.page = page;
nextDetails.pageSize = pageSize;
const pageType = output.findConfig('pagination.type') || 'page';
const pageMax = output.findConfig('pagination.max') || 200;
const start = page * pageSize;
const end = (page + 1) * pageSize;
if (pageType === 'offset') {
return [null, offsetStrategy(page, pageSize, start, end, pageMax)];
} else if (pageType === 'page') {
let startInd = parseInt(output.findConfig('pagination.page.startindex'));
if (isNaN(startInd)) { startInd = 1; }
return [null, pageStrategy(page, pageSize, start, end, pageMax, startInd, isCursor)];
} else if (pageType === 'cursor') {
let offset = nextDetails.offset;
let cursor = nextDetails.providerNextPage;
if (!util.notNull(offset)) {
offset = nextDetails.page * nextDetails.pageSize;
}
const nextPageKey = output.resource.nextPageKey;
if (!util.notNull(nextPageKey)) {
return ["unable to use cursor: no nextPageKey"];
}
return [null, cursorStrategy(page, pageSize, start, end, pageMax,
cursor, offset, nextPageKey)];
}
};
exports.createStrategy = (output) => createStrategy(output);
// An offset strategy asks the backend for a particular offset and
// limit, as often needed until all results are in. Offsets are always
// zero-based, so an offset of 0 is the same as asking for no offset at
// all.
const offsetStrategy = (page, pageSize, start, end, max) => {
let offset = start;
let limit = Math.min(max, end - offset);
let lastHeaders = {};
let hasMore = true;
return {
hasMore: (objs, data, cb) => {
lastHeaders = data.headers;
// TODO : check response header or something?
hasMore = (data.body || []).length >= limit;
objs.push.apply(objs, data.body);
offset += (data.body || []).length;
limit = Math.min(max, end - offset);
return cb(null, hasMore && offset < end);
},
input: () => ({ queryParameters: { page: offset, pageSize: limit } }),
headers: () => lastHeaders,
cursor: () => {
if (!hasMore) return null;
return ({
page: page + 1,
pageSize: pageSize,
offset: offset
});
}
};
};
exports.offsetStrategy = (page, pageSize, start, end, max) => offsetStrategy(page, pageSize, start, end, max);
// A page strategy asks the backend for a particular page and pageSize,
// possibly multiple times, finding the optimal page and pageSize every
// request. The 'startInd' argument indicates which index the backend
// thinks is the first record, and is usually 1 (for 1-based-index
// backends) or 0 (for 0-based-index backends).
const pageStrategy = (page, pageSize, start, end, max, startInd, isCursor) => {
let offset = start;
let optimal = optimalPaging(offset, end, max);
let lastHeaders = {};
let hasMore = true;
// non-cursor has been decremented
let incrementNumber = isCursor ? 1 : 2 ;
return {
hasMore: (objs, data, cb) => {
lastHeaders = data.headers;
// TODO : check response header or something?
if(util.notNull(data.body) && Array.isArray(data.body)) {
hasMore = data.body.length >= optimal.pageSize;
objs.push.apply(objs, data.body.slice(optimal.slice));
offset = optimal.page * optimal.pageSize + data.body.length;
optimal = optimalPaging(offset, end, max);
return cb(null, hasMore && offset < end);
}
return cb(null, false);
},
input: () => ({ queryParameters: shiftIndex(optimal, startInd) }),
headers: () => lastHeaders,
cursor: () => {
Iif (!hasMore) return null;
return ({
page: page + incrementNumber,
pageSize: pageSize,
offset: offset
});
}
};
};
exports.pageStrategy = (page, pageSize, start, end, max, startInd, isCursor) => pageStrategy(page, pageSize, start, end, max, startInd, isCursor);
// Find a pageSize that maximizes:
//
// min(end, (pageSize * (page + 1)))
// where page = floor(offset / pageSize)
// , pageSize <= max
//
// Where multiple pageSize values have the same score, we want to choose
// the smallest one.
//
// For example, if offset=55, end=75, max=10, the optimal pageSize is
// 9 which returns items 54-62: 8 of which are values we care about,
// and not (as one might naively suspect) 10 which gets items 50-59,
// of which only 5 we care about.
//
const optimalPaging = (offset, end, max) => {
// start with max size, and go downward until
// TODO : use prime factorization combinatorics instead?
let pageSize = Math.min(max, (end - offset) * 2);
let bestPage;
let bestPageSize;
let maxCapture = 0;
while (pageSize > 0) {
let page = Math.floor(offset / pageSize);
let capture = Math.min(end, pageSize * (page + 1));
if (capture >= maxCapture) {
maxCapture = capture;
bestPage = page;
bestPageSize = pageSize;
}
if (maxCapture >= pageSize + offset) break;
--pageSize;
}
return {
page: bestPage,
pageSize: bestPageSize,
slice: offset - bestPageSize * bestPage
};
};
exports.optimalPaging = (offset, end, max) => optimalPaging(offset, end, max);
const shiftIndex = (optimal, startInd) => {
return {page: optimal.page + startInd, pageSize: optimal.pageSize};
};
exports.shiftIndex = (optimal, startInd) => shiftIndex(optimal, startInd);
// A cursor strategy sends a cursor to the backend, which is a
// continuation token that resumes record retrieval at some offset
// indicated by 'cOffset'.
//
const cursorStrategy = (page, pageSize, start, end, max, cursor, cOffset,
nextPageKey) => {
let offset = cOffset;
if (offset > start) {
// boo, restart from the top
offset = 0;
cursor = null;
}
let slice = start - offset;
let size = Math.min(max, end - offset);
let lastHeaders = {};
return {
hasMore: (objs, data, cb) => {
lastHeaders = data.headers;
extractCursor(data, nextPageKey, (err, gotCursor) => {
Iif (util.notNull(err)) { return cb(err); }
cursor = gotCursor;
Eif(util.notNull(data.body)) {
offset += data.body.length;
//If the results is an array then add the results to the obj,
//if its an object then just insert the object to objs
if(data.body instanceof Array) {
objs.push.apply(objs, data.body.slice(slice));
} else {
objs.push(data.body);
}
}
if (start > offset) { slice = start - offset; }
else { slice = 0; }
size = Math.min(max, end - offset);
return cb(null, util.notNull(cursor) && offset < end);
});
},
input: () => ({ queryParameters: { pageSize: size, nextPage: cursor }}),
headers: () => lastHeaders,
cursor: () => {
Iif (!util.notNull(cursor)) return null;
return ({
page: page + 1,
pageSize: pageSize,
offset: offset,
providerNextPage: cursor
});
}
};
};
exports.cursorStrategy = (page, pageSize, start, end, max, cursor, cOffset, nextPageKey) => cursorStrategy(page, pageSize, start, end, max, cursor, cOffset, nextPageKey);
// Extract a cursor from a response. The 'nextPageKey' determines
// where in the response to find the cursor, and can be
//
// * `header.<header_name>` : found as a header
// * `body.<body_location>` : found as a body value
// * `body.<body_location>?queryParam : query param on a body value
//
const extractCursor = (data, nextPageKey, cb) => {
if (nextPageKey.startsWith("header.")) {
nextPageKey = nextPageKey.substr(7);
cb(null, (data.headers || {})[nextPageKey]);
} else if (nextPageKey.startsWith("body.")) {
nextPageKey = nextPageKey.substr(5);
let q = nextPageKey.indexOf('?');
let query = null;
if (q !== -1) {
query = nextPageKey.substr(q + 1);
nextPageKey = nextPageKey.substr(0, q);
}
let val = _.get((data.originalBody || data.body), nextPageKey);
if (util.notNull(query) && util.notNull(val)) {
val = url.parse(val, true).query[query];
}
cb(null, val);
} else {
cb(util.failure(500, `Unknown nextPageKey format: ${nextPageKey}`));
}
};
exports.extractCursor = (data, nextPageKey, cb) => extractCursor(data, nextPageKey, cb);
const convertMerged = (output, strategy, objs) => {
let headers = strategy.headers() || {};
headers["Elements-Returned-Count"] = objs.length;
let cursor;
if (util.notNull(cursor = strategy.cursor())) {
headers["Elements-Next-Page-Token"] = util.encodeJson64(cursor);
}
return {
body: objs,
headers: headers
};
};
exports.convertMerged = (output, strategy, objs) => convertMerged(output, strategy, objs);
|