explain this 
const mongoose = require("mongoose"); const utils = require("../../utils/utils"); const _ = require("lodash"); function update(docService, doc) { let Model = docService.getModel(); let docEntry = new Model(doc); docEntry.isNew = false; docEntry.save(function (err, e) { if (err) { return console.error(err); } }); } async function updateFields(docService, filter, fieldValues) { let Model = docService.getModel(); try { const result = await Model.findOneAndUpdate(filter, fieldValues); return result; } catch (error) { throw error; } } async function setMessage(docService, doc, msg) { await updateFields(docService, { _id: doc._id }, { txMessage: msg }); } async function markAsProcessed(docService, doc, externalId, msg) { let filter = { _id: doc._id, }; let fieldValues = { externalId: externalId, txStatus: "PROCESSED", txMessage: msg ? msg : "PROCESSED: Saved with External Id " + externalId, }; return await updateFields(docService, filter, fieldValues); } function markAsPermanentlyFailed(docService, doc, msg) { let filter = { _id: doc._id, }; let fieldValues = { txStatus: "PERMANENTLY-FAILED", txMessage: "PERMANENTLY-FAILED: " + msg, }; updateFields(docService, filter, fieldValues); } const MAX_ATTEMPTS = 3; function markAsFailed(docService, doc, msg) { let filter = { _id: doc._id, }; doc.txAttempted++; let fieldValues = { txAttempted: doc.txAttempted, }; if (doc.txAttempted >= MAX_ATTEMPTS) { fieldValues.txStatus = "PERMANENTLY-FAILED"; fieldValues.txMessage = "PERMANENTLY-FAILED: " + msg; } else { fieldValues.txStatus = "FAILED"; fieldValues.txMessage = "FAILED: " + msg; } updateFields(docService, filter, fieldValues); } async function markAsSkipped(docService, doc, msg) { let filter = { _id: doc._id, }; let fieldValues = { txStatus: "NEW", txMessage: msg ? msg : "SKIPPED: Due to missing prerequisites", }; return await updateFields(docService, filter, fieldValues); } async function sendDocuments( job, interfaceFunc, docService, docList, tranName, successFunc, failureFunc, skipFunc ) { job.logInfo( docList.length + " " + tranName + " document" + (docList.length > 1 ? "s" : "") + " being sent..." ); let success = true; for (let doc of docList) { // @@ Seems like sometimes we are calling this with a Mongo document object and sometimes as a plain JSON object // @@ Fix it! This is clumsy let d = doc.toJSON ? doc.toJSON() : doc; job.logDebug("Sending " + tranName + " (" + d.tranType + ") ..."); job.logDebug(utils.recursiveStringify(d)); // try { let r = await interfaceFunc(d); if (!r) { success = false; } else if (r.status === "SUCCESS") { if (successFunc) { await successFunc(doc, r.internalId); } else { // Passing the internalId of the remote system's record as externalId in our records // Passing the DB document doc intentionally and not the JSONified version d await markAsProcessed(docService, doc, r.internalId); } } else if (r.status === "SKIP") { if (skipFunc) { await skipFunc(doc, r.message); } else { // Passing the DB document doc intentionally and not the JSONified version d await markAsSkipped(docService, doc, r.message); } } else { if (failureFunc) { await failureFunc(doc, r.message); } else { // Passing the DB document doc intentionally and not the JSONified version d markAsFailed(docService, doc, r.message); } success = false; } } job.logInfo("Done"); return success; } /* To get the count of all documents across all models*/ async function getAllCount(docService, filter) { let Model = docService.getModel(); try { const result = await Model.countDocuments(filter); // console.log(result); return JSON.stringify(result); } catch (error) { throw error; } } /* To get the details of all documents across all models */ async function getAllDetails(docService, filter, fields) { let Model = docService.getModel(); return Model.find(filter, [ ...fields ]) } /* To get the details of all Processed documents across all models */ function getAllProcessed(docService, filter, skip, limit, fields) { console.log("skip:", skip, "limit", limit); let Model = docService.getModel(); return Model.find(filter, [ ...fields ]) .skip(skip) .limit(limit) .sort({ createdDateTime: -1 }); // In Ascending order } /* To get the details of single document for JSON print */ function getSingleDetails(docService, filter) { let Model = docService.getModel(); return Model.find(filter); } /* To get the count of processes documents across all models */ async function getProcessed(docService, filter) { let Model = docService.getModel(); try { let result = await Model.countDocuments(filter); // console.log(JSON.stringify(filter)) return JSON.stringify(result); } catch (error) { throw error; } } module.exports = { markAsProcessed: markAsProcessed, markAsFailed: markAsFailed, markAsPermanentlyFailed: markAsPermanentlyFailed, sendDocuments: sendDocuments, update: update, updateFields: updateFields, setMessage: setMessage, getAllCount: getAllCount, getAllDetails: getAllDetails, getSingleDetails: getSingleDetails, getProcessed: getProcessed, getAllProcessed: getAllProcessed, };